This assumes, of course, that people will not import Pit Bulls from, say, Oakland or San Jose. It would never happen. And people certainly wouldn't drive to Oakland to pick up a Pit Bull if they couldn't find any in Frisco.
But get this: Breeders of show dogs recognized by a kennel club would be exempt. Translation: if you're rich and you live in San Fran, you're probably a responsible dog owner; if you're poor and you live in San Fran, you're probably an irresponsible dog owner.
I leave the joke-making to the meta. I have work to do.