I'd imagine it's like any other prohibited material or substance; drugs, counterfeit goods, pirated works, etc. Of those implicated, a small minority are involved in production, while the vast majority are simply consumers.
I assume you are willing to at least acknowledge the existence of the latter group; people who simply consume child pornography and play no part in the production, or any sexual abuse of children for that matter.
Do you really think it is appropriate to ruin these peoples' lives — splash their names across the newspapers, lock them in jail, destroy their career, their relationships, make them a social pariah — for mere possession of child pornography? For having a certain sequence of 0s and 1s on their computer?