Stan Freberg (RIP, Stan) had a funny take on this issue in a sketch called "Elderly Man River," in which he tried to sing Ol' Man River while a censor kept stopping him to correct his grammar and the politically incorrect lines.

http://freberg.westnet.com/text/elderly_man.html

That said, I have worked as a proofreader/editor, and am also an amateur singer/songwriter and performer.

Songs are an honest expression of feelings written in the moment, not a business or professional treatise. They are also frequently written from the point of view of a character who is not necessarily the writer or singer. They are a form of monologue, or dialogue, which reflect the feelings of person who does not necessarily speak in perfect English. Call it poetic license, if you will.

I would no sooner change the language or grammar in a song lyric than I would try to correct James Joyce or correct a speech in a play.

Or, as Tom Lehrer sang, "It sounds more ethnic if it ain't good English, and it don't even gotta rhyme..."