I'm against BSL and I think I have raised some good points - none of which have been addressed by these people. Why do the dogs need to stay dog-aggressive if their primary purposes are conformation showing, weight-pulling and family pets? And why is this an unreasonable question to ask?
Good catch on the "common code" remark, HairOfTheDog.
The easy answer to your question is that they ~don't~ have to be aggressive to do legitimate sports. They are still aggressive because people are very much still breeding them for the purpose of fighting. And aggression is not so precisely compartmentalised as they'll tell you it is. There's a LOT of loose cannons born in the dangerous realm of breeding for 'gameness'.
I'm not for banning any breeds either... I think it's impractical in the real world where anybody can breed a dog that may or may not be part or full pit bull. We can't even win the drug war, and we can at least definitively identify the drugs involved. My only goal is to educate people who do want pets that pit bulls are a lousy choice compared to all the great companion breeds out there. If good people stopped trying to tell us they are just as cuddly as a teddy bear, stopped trying to promote the lie that they are safe, then at least we wouldn't have innocent people maimed when their nature comes out... When we saw a pit bull, we'd know what it is, mourn what it is... investigate cruelty against it, but not try to tell a mom she should let it play with her kids.