To be clear, the "jailbait" forums that Reddit tolerated for years were not officially tolerating material that would be clearly illegal by US standards. If they had been, Reddit would have shut them down out of self-protection. Possessing outright child pornography is a strict-liability offense.

They were, however, posting "upskirt" photos showing children's underwear. They were posting pictures of children at the beach in swimsuits. They were, in short, posting photos of actual children, where the photos had been selected for being sexually appealing to pedophiles.

Not drawings of fictional characters; real kids, selected for hotness to pedophiles.

I think that's something about which site owners can very reasonably say, "Even if it is not a criminal offense, we think it's wrong and we don't want it on our site" without anyone needing to be concerned about censorship of ideas.