Why is that? I’d consider this equivalent to the (justified) banning of Nazi imagery in countries like Germany, Austria, Norway, Australia, etc.
No one is harmed by a piece of paper or cloth with a symbol on it, but harm happens because of the symbol’s implications.
“Authorized” AI-generated or illustrated depictions of CSAM validate the sexualization of children in general, and should not be permitted, in my opinion. If it enables real CSAM to continue, then AI-generated content is not victimless, and therefore I don’t think these hypothetical individuals going to prison for it are necessarily innocent.