They are probably not bred well by responsible breeders. I think a lot of backyard breeding goes on with pit bulls. A responsible breeder would not breed for agression in a dog. I think this breed is bred for the most part by yahoos who just want a vicious dog.
When I walk my little corgi, I am always armed in case a pit bull should attack him (or me).
Actually, what you say is probably true, but you don’t necessarily need breeding to make a dog mean.