If I were a bike thief, I'd travel to the nice part of town to find a bike to steal -- partly because the bike might be nicer (although that wouldn't matter if it's only going to be sold for scrap), and partly because the residents are more likely to leave them unlocked. Plus, if I live in a bad part of town already, I'd be less likely to steal a friend's bike if I went somewhere else.

Besides, I'm less and less surprised that even the crappy bikes get stolen, if only because of how crappy the bikes are that I see being ridden regularly.