Why if pitbulls and other so called dangerous dogs are banned is no one up holding the law?

DONT GET ME WRONG I LOVE PITBULLS! I know a couple of real softies but everyone goes on in the news about pitbulls mastiffs and akitas being banned due to agressive tendancies. Yet is no one stopping them being bred if owning one against the law why arent owners arrested? Why does it seem that the law doesnt abide concrete to everyone? I love all dogs but can see both sides of the dangerous dogs arguement when my own dog was savaged by a akita 2 years ago but I also know wonderfull staffs so where do u draw the line between law and the right to live.