According to Sunni and Shia (?) teachings, hadith is not directly protected by Allah. There's a lot of evidence that many ahadith have been fabricated out of the temptation to establish a rule on ones own or just because someone told alike, and a human system to evaluate whether the hadith is probably authentic had to be established.
We received the Quran and we believe not without reason that it has been preserved as Muhammad (p.b.u.h) has received it.
The grading of a hadith has become important because the entire legal system in Islam (Sharia) had been established. The reliability of ahadith had to be fixed because the message of the Quran only left too many cases not exactly defined, so that hadith had to be used, and a clear classification was necessary to come to a system of legal security (I.e. that judgement was not arbitrary).
Thus, hadith is almost handled like a divine decree - but there is no final certainty that it really is.