Lots of things are dangerous, we don't ban all of them.
Staffs in particular are an issue with culture. Like most dogs, they are largely a fashion accessory, just in this case it attracts people who seem to like to look 'hard' and having an aggressive dog is part of that.
The laws with regards to dangerous dogs has a huge, huge hole in it and that's personal responsibility. One of the puppies from my dog was put down as it bit a kid. What wasn't considered was that the kid in question had spent 20 winding the dog up, poking it,chasing it and generally being a right little ****. Hardly fair to the dog in my opinion.