I'd have thought that there is a lot to do with the psychology of the owner. You don't get a pitbull to look pretty and keep you company, you get it to look vicious and scary. I would say that the sort of person who wants a vicious dog is also the sort of person who will train, or at least allow, it to be vicious. In Britain pitbulls are banned because they were almost exclusively used for dogfights, which are illegal, so without the dogs there are no fights.