Keep in mind the only bad thing about pitbulls is they're bull-headed: They'll barrel in to things and can be rough simply because they're hyper little weirdos, not violent killing machines. Roughness can be misconstrued as violence, even in humans (ask me how I know!).
I say all that to mean ... maybe also befriend the dog regardless of what happens. If you're nicer to him than the "trashy" owners, he'll listen to you better and maybe be nicer to the stuff he thinks is yours regardless of what the owners do.