Pit Bulls dont let go because of pride for their job and protection of its master. Despite what a lot of people beleive they WERE NOT bred to fight. They were bred to help capture large livestock such as bulls and even bears so picture this.....
A group of men out in the wild hunting bears and bulls to capture or kill, even with several shots that animal is going to take a while to hit the ground and die. In the meantime he would thrash around, charge and try to kill anything in its path. Thats where the pit bull comes in! He would grab hold and keep his master from harm until he was commanded to let go or felt the life drain away. He did it out of pride, honor and servitude to his master.
Humasn made them they way they are and I find it terribly sad that since they are no longer needed they have been diminshed to such a low level in the public eye.
That is also why they are my favorite breed, they are so stoic and eager to please.