My own pet theory... it's not so much about WRITING the song as PERFORMING the song. Performers are often (not saying always) more intimately identified - in their own mind & the mind of the audience - as the narrator or primary character in a song. Most of us do not wish to regularly put ourselves in the mindset of someone whom we abhor in order to get a good performance, and do not want to give the audience the impression that we actually support some ideas.

Parodies, satire, songs about witnessing evil, or songs where the evil-doer gets his/her due (even if sung in first person) are a different sort of thing - you're not putting yourself out there as if you're promoting the act. But if you sing about something like killing innocent people, sing as if you really mean it, chances are you'll at least get watched closely by the FBI and maybe locked up "for the public good".

A novel, in addition to having much more time to explore the subtleties of a mindset, also tends to be more dissociated from the author. Perhaps even written poetry can have that same sort of separation - the reader deals with the words as he/she wishes, without directly involving the author. A song, which is intended to be performed, is an immediate and involving experience that links performer & audience. That's part of why songs might arguably be more powerful than poems or prose, but also why many people avoid some topics.

~Val