A: don't know about "should", but they can't. as long as the words in the lyrics follow the same rhythm/structure as the original, that's a cover song and using a whole bunch of text with another name on would require plenty of authorizations on the part of the original owner (suppose nevermore hit it big and people associate those lyrics with the band... that's a large sum of money definitely not going the appropriate way), which, being in the case in point simon & garfunkel, i daresay is not exactly the most approachable, down-to-earth subject on this earth.
Q: how about explicit rip-offs of someone else's music? think the offspring's "why don't you get a job?", which shares more than a passing resemblance to the beatles' "obladì obladà" (sp). is that a reverential quote? a hoax? coincidence?