The problem remains that the second lemmy.world allows content created by someone underage to federate onto their server, they probably have some legal responsibility regarding that data. And if there is personal information in there, it gets tricky pretty fast in some jurisdictions.

LW don’t want legal problems, that’s literally all there is to this.

Whether LW can enforce their ToS on remote users is a different question, and even if the answer to that is “no” then they could still include that clause in every single LW community’s rules.