[Blasted Child]

Yeah, Zhukov, that's without doubt the reasoning behind this - at first - seemingly absurd law.
However I think that if you're venturing into the realm of hypotheses - this could inspire people to do this and that, one could also claim that it could in fact prevent the same crimes, simply by letting perverts live out their fantasies (that is, jerk off) without actually harming a real human. It's really hard to know, and I'm pretty sure there hasn't been any proper research to provide evidence in either direction.

Also, it's interesting that texts portraying the same scenery are not prohibited. Lolita will probably never be considered illegal material. But if pictures can influence people to commit certain deeds, surely a well-written novel can do the same?