The problem with pitbulls (in my opinion) is that in urban areas, you see a lot of gang-bangers/thug types with these dogs. They are obviously using these dogs as a type of "status" symbol, to show how tough they are. And quite frankly, they are probably training these dogs to be vicious. Now not everyone, but it's more common that you might think.
I live in a not so great part of Chicago, pitbulls and gangs a plenty. I have to be honest, when I'm walking my little guys, I get nervous when I see a pit. It's a shame really, because I do realize it has to do with the owner, but bad owners give these dogs a bad reputation.