Not all child pornography is created by the rape of children. Classification of CP can include fictional stories, artistic depictions (often anime style drawings) and 'child models' in suggestive poses.
Rape is a crime and should be treated as such. Fictional CP shouldn't in the same way we don't make watching videos of sex, movies containing murder, etc illegal.
The "they weren't abusing children themselves" argument could equally apply to consumers of child pornography. If law enforcements argument that distribution and viewing of CP is equivalent to re-abusing the victim then it should equally apply to them.
Rape is a crime and should be treated as such. Fictional CP shouldn't in the same way we don't make watching videos of sex, movies containing murder, etc illegal.