Well, the system works on a basis of preusmption of innocence (in legal terms). So should people. Granted, they don't - but you can;t regulate peoples thoughts either.
Likewise, how much damage could/would song lyrics actually do? Generally speaking, I don't think they're regarded as a valid source of information by most people....i.e. they won't form opinions based on them, although they may reinforce their own.
I think, though, the crux is that you need to have total freedom of expression in art - even if that means stuff which is libellous, or in some way 'dangerous' (or just plain crap), gets through. Society sets clear boundaries in what is acceptable - and there is a legal system which enforces these. The risk of censorship is that you filter out the statements that pose valid questions - that's more dangerous than allowing the odd bit of slander to slip out, IMO.
NB: I recently read somewhere a not that there isn;t much art being done which tackles modern issues of conflict - i.e. there's not a Guernica for Iraq, etc. Maybe music is becoming the new medium? Or maybe it always has been? (since Vietnam, at the least)
EDIT: oh, RE the rape analogy - I think a comparison is, is it better to persue every rape accusation, or to only persue those which seem valid? This is itself an issue - false accusal is not exactly unheard of - but the single risk is the danger posed when you ignore that one true case that didn't sound real. I think.
