One major problem i personally have with those who say we take the Quran only is: That Quran has been brought to us through a sane narrator chain with a high degree of tawatur, which is (or at least should be according to definition) the same case for Sahih Hadith. And riwayat al-ahaad in case of Quran have always been rejected!
Note that as @makzimus said there are many groups of Quranists some are more or less "organized", so their beliefs and opinions can differ a lot from each other!
I won't quote Shia view here because i couldn't find it -so far- at least not in Arabic or English. I only found some statements of scholars who had some baseless accusation on the relationship between Shia and Quransits!
Lets start with some Fatawa
Before listing some fatwas with short comments i would like to quote a story narrated by Sheikh al-Kubaysi in a lecture at a Malaysian University:
a tabi'i or maybe tabi' at tabi'yn who told a friend -advising him-, that he heard a Hadith telling that the Messenger of Allah (Peace be upon him) used to like such and such, the listener answered him saying: But i don't like it, so the other man -who narrated the Hadith or told him about the preferences of our Messenger (Peace be upon him)- took his sword and killed him because of blasphemy.
I think this story is very ambivalent as it seems to allow or encourage arbitrary law and shows an exaggerated interpretation of the Sunna of the Prophet (Peace be upon him). But this already should show the importance for Sunna to Sunni people. Unfortunately most of my sources are in Arabic language!
- In this fatwa the scholar qualified a man described as rejecting Hadith and Sunna and interpreting Quran as he like or understood as out of the folds of Islam and Kafir. The scholar referred primarily to the Verses (4:80), (59:7), and (16:44) and to the Hadith from Sunan abi Dawod which i quoted here.
- An other similar fatwa with similar references in addition to (4:65) and (53:3-4), but with the addition that this person shouldn't inherit from any Muslim, his wife should be separated from him by faskh and he should be taken to the authorities/court to examine if he is an apostate. And he added an quotation of Yahya ibn abi Kathir (from Sunan a-Darimi):
السنة قاضية على القرآن، وليس القرآن بقاض على السنة
Where قاضية shouldn't be understand as judging or ruling over, but as explaining.
So this statement should be understood as follows (My translation): The Sunna is explaining the Quran, while the Quran doesn't explain Sunna! (My reference for this explanation is this fatwa) If we understood it literally then we would set Hadith much higher then Qur'an and that couldn't go ahead with the laws of Shari'a!
- In this Fatwa the scholar was asked about the Islamic stance on somebody who rejects a sahih Hadith.
Conclusion: If he rejects it because of nafs or hawa he would be considered as kafir!
If he rejects it because he deny it being the word of the Messenger of Allah (Peace be upon him) then his statement should be examined academically!
So the last sentence could be apply for a lot of Muslims and scholars: For example i read a commentary of ibn al-Majushoun a Student of Imam Malik, who has been asked about a matter which the maliki school rejects even if there have been an athar (Hadith or only teaching of a sahabi) about it in the Muwata' with the Answer: "We have quoted it and rejected it so that people know, that we knew it and rejected it" ...why? Because they either had a better/stronger narration or something based on the osol of their madhab.
فقيل لـابن الماجشون: لمَ رويتم الحديث ثم تركتموه؟ قال: ليعلم الناس أنا على علمٍ تركناه.
And you may find many Ahadith even from sahih al-Bukahri or Muslim which were rejected by fiqh scholars, why? Because Hadith scholars: are only transmitters in first place, while fiqh scholars need to examine these narration and how they could refer to shari'a, so they examine the sanad (narrator chain) an matn (content) and whether it goes along with the Quran and the maqasid (goals) of shari'a!
But in the belief of ahlu-l-Hadith especially beginning with Imam Ahmad, one shouldn't record a Hadith unless one believes it so they must believe and apply it if possible!
Back to the fuqaha': i read a long paper about the method of Imam Malik to chose or reject Ahadith, so for example: Assuming he found two contradictory Hadith, one narrated by a Sahabi who very early became a Muslim and one from a trustworthy Sahabi who became Sahabi a few years before the death of the Prophet (Peace be upon him) he could deduce that the saying of the first Sahabi might have been abrogated and the later knew it especially if this Sahabi has been known to have passed much time listening and following the Prophet ((Peace be upon him), if he couldn't find a way to deduce a consensus between both narrations and meld them. Imam a-Shafi'i -as the first one who wrote about 'ilm Osol al fiqh- has described his method (or methods of verifying and choosing Hadith if there contradictory narrations) in his books like ar-Rissala.
So rejecting a Hadith needs a lot of research, but if you do so and came to the conclusion that this Hadith maybe a fabrication or have some weaknesses or you couldn't meld it with a sane narration or found contradictions to Quran after a certain effort this is allowed and a kind of ijtihad!
But on the other hand rejecting a whole book wouldn't make sense, because for example one of the first recorded Hadith collections is al-Muwatta', and even if later scholars didn't copy it and add it to their books, they have heard it from students of Imam Malik and you may find among the ca. 700 Ahadith/Hadiths (By Hadith i mean Hadith in it's most accurate definition: saying and doings of the Messenger of Allah (Peace be upon him) and not saying or fatwa of Sahabi or Tabi'i which normally is called Athar or Khabar) you may find more then 500 hadiths in Sahih al Bukhari and a similar amount in any of the 6 Books you described as the "Sihah a-Sitta" and Musnad Ahmad etc.. And you may find Ahadith of Imam Malik which are not covered in the Muwatta' at least not in that one we refer to (Muwatta' Yahya ibn Yahya al-Laithi) which is the last Muwatta' which was approved by Imam Malik.
So you may see the contradiction if we reject a book then we reject a Hadith we already accepted in an other, yes maybe the last narrator in the chain -the student of Malik- maybe weak, but if the matn is the same, what would that mean?
A Hadith which i found quoted in an other fatwa is speaking about people who reject prohibition or orders of the Prophet (Peace be upon him) because it's not in the Quran. This was narrated in Jami' at-Tirmidhi, Sunan abi Dawod, Sunan ibn Majah and Musnad Ahmad and qualified as sahih.
to complete my answer on Q2 i still need to answer:
Aren't there Authentic/Sahih Ahadith in those other books?
Well the answer here is based on my own opinion: I think you may find -some- sahih hadiths in any book of hadith even in the not common works like Jami' (Sufyan ibn 'Uyaynah if it still exits) in the works of Abdullah ibn al-Mubarak in the sahih of ibn Shaybah, the sahih of ibn Hibbaan, Musnad ash-Ashafi'i and abu Hanifa (at least in Muwatta' Muhammad ibn al-Hassan a-Shaybani you may find a lot of evidences of the Hanafi school beside the ahadith of Imam Malik) and why not in the four major Collections of Shia? etc. which are not in the major Hadith Collections. But for some reasons the both sahihs of al-Bukahri and Muslim have gained the highest value among Sunni scholars. Honestly you must also take into account some political reasons for example during the 'Abbasid dynasty they supported for a while the Hanafi Madhab, Imam Malik (when they wanted to make the Muwatta' the 2nd source of shari'a) then the Mu'tazillah and to some extent shi'a then again ahl-Sunnah ... so some scholars have had a more or less good relationships to the authorities then others and therefore their work might have reached a larger audience.
So far i gave an answer to Q2 and the major Question on the ruling on them and whether they are out of the folds of Islam! I will restrict on giving some brief additions to makzimus's great Answer!
Q1. Why do Quranists reject all the Hadiths?
Because they take Verses like (5:48)
And We have revealed to you, [O Muhammad], the Book in truth, confirming that which preceded it of the Scripture and as a criterion over it. So judge between them by what Allah has revealed and do not follow their inclinations away from what has come to you of the truth. ...
(7:2) , (15:9) and
Allah has sent down the best statement (=Hadith)
literally. Ignoring Verses from the Quran like (3:31), (4:59), (4:80), (33:71) and (59:7), where Muslims are asked to follow the Prophet (Peace be upon him). From this point of View Quran is considered as guarded and safe while anything else -namely Sunnah- is not! So they pretended that our Messenger (Peace be upon him) only applied the Quran in his life. But this is only true if we consider the addition of ahkam which explained and expanded the ahkam the Quran literally gave!
How can they extract many rulings/understandings that our Ai'mah have extracted using Quran o Sunnah?
well as you could see in the example of prayers, one could deduce that there are five prayers, but only the Sunna gives clarity, as one can get a couple of exact timings of prayer from the Quran but there's no Verse that really say that there are 5 prayers and even whether the number is odd -according Quran only- could be discussed! But i read fatawa were some Quranists deduced 3 prayers and others 2 prayers only because they toke the timings literally. This means they are somehow playing with the ahkam as they interpret Quran as they like. This would also mean they have no real osol to build on the ahkam!
And here applies the logical rule: Ex falso quodlibet -> From a wrong assumption (in this case interpretation) you can conclude anything (right or wrong).
And according to Arabic Wikipedia some of them reject the naskh!
Wikipedia references: English and Arabic.
And Allah knows best!