And these sorts of defederations may also serve as a "hey, do something about this" smack to the defederated instances. Provided that lemmy.world and sh.itjust.works don't actually want to have trolls like that among their users they could implement mechanisms to make it harder for them to sign up.
If someone wants to deliberately run a 4chan-like free-for-all instance, that's fine, but I expect nobody's going to want to federate with it.