Make causing actual harm to an actual child illegal. Make profiting from it (eg selling pictures of said actual harm) illegal. Make possessing pictures not illegal, but something that will get you asked where the pictures came from.
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But really it is about thoughtcrime. Society (or at least politicians) have decided that having certain interests/desires is wrong (or rather, that those people are creepy and dangerous and can't possibly have the self-control to not fully act on said thoughts). And so anything that may indicate someone having said thoughts is made illegal, even if it doesn't hurt anyone, and even if it indicate that they know not to act in harmful ways.
Can't someone accused of selling child porn claim it's fake just as easily as someone simply accused of possessing it?
I would think that sellers having ties to producers would have a harder time arguing they were selling fakes. And if they don't have those ties -- if they're not supporting the people causing the harm -- then their actions are only illegal for the sake of simplifying the prosecution's work. Which means their likely being able to get off at trial is less of an issue.
* In this scenario it's something creepy but harmless. In another it might be fake guns or fake drugs * *
* * Consider the implications of outlawing bags of sugar.