In our local song circles when someone asks, "Who wrote that song?", they are generally asking because they really like the song, and they'd like to know who wrote it so they can find out about some other good songs the same person wrote...or maybe find out what CD it's on...or if it was written by the person who just played it, in which case they can compliment them for such good songwriting skills.

In any case, it's not a question that strikes me as offensive at all, unless it's accompanied by a look and a tone of voice that tells you they think the song sucked! ;-) (and that almost never happens)