Yes, they are part of the problem, but people aren't cruising the streets looking for a pimp or a trafficker, they are looking for prostitutes. The so-called "working girls" are just as culpable as the Johns, the pimps and the traffickers. To say anything less is just plain silly. Here's a crazy idea, why not actually enforce all the laws equally? Like it or not, prostitution is a crime in San Francisco and should be treated as such.
The problem in San Francisco (and many cities) is one of inequity. The governmental bodies refuse to enforce the law as ordered by the people. As a nation of laws we must be beholden to the will of the people as evidenced in our laws. If a law is unpopular does that mean that the law should go unenforced? No. It means that it should be enforced until such time as it is changed. To do anything else lessens the power of the remaining laws as people can then pick and choose which laws to enforce (or obey). That is the whole point of being governed by law. We can't pick and choose which laws to obey, nor should the government pick and choose which laws to enforce. If a law is unpopular amongst a majority of people, then let the people decide to change it.