Oh, they CAN get busted in SF, but they hardly ever do. And if they do, it's barely a slap on the wrist. Not exactly a valid comparison.
I once went riding with a policeman in a city not too far from there. He was stopped, talking with another policeman when a call came in about some teenagers who were apparently smoking marijuana. The one cop said, "We'd better get right on that." Then they continued talking about other things.