Exactly. And pictures of scantily-clad young teens do not CAUSE crime, either. They are, perhaps, a contributing factor, although I somehow doubt that:
(a) an otherwise normal citizen with no history of pedophilia will suddenly decide to molest a child because he sees a provocative photo, or
(b) a pedophile will decide NOT to molest a child because he can't look at provocative photos of children online anymore.
In fact, the incredibly unlikelihood of option (b) is why I see no reason for a law here. Stopping this admittedly disgusting practice will do absolutely nothing to stop violent pedophiles from harming children.
It will slow the need for more pictures of exploited children to be displayed. These children are being USED for sex and the people doing it should be punished. The web site owners are selling children and sex.
You cannot convince me that the legal need for these websites is more compelling than the need to stop sexually exploiting these children.