its a canon thing. I've drawn timeskip Yoko topless, but because she was underage before it, that got taken down too. And my friend drew a full clothed pic of May(pokemon), just with an erotic expression on her face and DA removed it. So it must be something they're really stict about.
Still, i think it is an inconsistency, many manga characters who claim to be underage have actualy bodies more feasible for a grown up adult, but just stating it is an underage character qualifies it for deletion here.