Muslims will pretty much always include some honorific after the names of prophets and other religions figures (such as PBUH, etc.) Why are people so careful to always do this every time? Is this based on some legal source, or is it simply out of respect?
Love and Respect are the main reasons...
There is some legal aspect to it - as is mentioned in the Hadith (At-Tirmithi, 3545):
It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “May his nose be rubbed in the dust, a man in whose presence I am mentioned and he does not send blessings upon me.”
There are other Hadiths with similar meaning... for further information see:
Love and respect is the ultimate reason for the honorifics for people that sacrificed their whole lives for us. There are also hadiths that encourage praising our Prophet (SAW) - not only by the person that has mentioned him, but also by the people that have heard his name being mentioned. I believe using an honorific after the names of other religious figures is derived from this.
The Messenger of Allah said: "Whoever sends one salaam (صلى الله عليه وسلم ṣall Allāhu ʿalay-hi wa-sallam) upon me, Allah will send ten upon him." —Abu Hurayrah
Allah and His angels send prayers on the Prophet: O ye that believe! Send ye prayers on him, and salute him with all respect. —Al-Ahzab