It has been a while since I went to 4chan, but for /b/, besides very few exceptions like child porn, the rule was "no rules, it also applies to mods".

So they can allow the worst kind of hate speech, porn and gore, but ban a harmless meme because mods find it annoying. So the only rationale from banning images from "Cuties" may be "because mods don't like it".

4chan already bans anime/cartoon style deciptions of underage content, so it's not really surprising that they ban this too