Most people with unfulfilled sexual fantasies are able to legitimately fulfill at least some of their fantasies - I may not be able to sleep with my favourite celebrity, but I can sleep with somebody I'm attracted to. Paedophiles (or at least exclusive paedophiles, i.e. those not also attracted to adults) don't have that opportunity without abusing children, so the logic is that it's better to try to kill the desire altogether (vs. the other side that non-real pornography gives them an outlet for that desire so as to avoid abusing children or looking at real images).

Personally I think you're right and that simulated images shouldn't be banned, but I also think I don't know nearly enough about psychology to know if I'm right.

edit to your edit about knowing real from fake images: Again, I'm no expert (this time regarding the technology rather than psychology), but there are more issues than your solution fixes. That would protect someone who was creating the images, but let's say you're a paedophile who wants to look at generated images but doesn't want to look at real images, how will you know wherever you're getting them from is telling the truth? You could be told "these are really realistic computer images" and actually be watching a real child get abused. And if the police investigate that viewing paedophile, without having models/etc uploaded (as he wouldn't have access to them) how do they know if they've discovered real or fake porn? Well I guess psychology does come into play here as I've no idea if there are paedophiles who would be willing to watch something realistic enough to not know the difference but wouldn't be willing to watch real images. Though I suspect there probably are.