I think the fade out thing exists also because of the lack of space on vinyl records. If would add 15 to 20 seconds or even more for each song to finish properly. Genesis records were already quiet long, maybe they thought it would compromise the sound too much.
Of course, this does not apply to CAS, I think it's only a matter of "we don't know how to finish".
Concerning Abacab, because it was a studio jam, maybe they didn't know how to end the song when they recorded it, and it only came later, by rehearsing for the tour with Daryl and Chester. But it's just a theory.