It depends on the jurisdiction. In the United States, we have the DMCA which has been weaponized by content creators and publishers, but we also have a "safe harbors" provision to the DMCA that is supposed to protect online service providers from being liable for copyright infringement based on the actions of their users - as long as they meet certain provisions and restrictions and perform certain duties and dilligence. And yet even with that in place, it does not stop content providers from suing service providers and forcing those service providers to incur the pain and expense of mounting a legal defense.

I am pretty sure that Lemmy.world admin team are European and that the instance is hosted somewhere in Europe, so they would have their own jurisdictional laws to follow.

TL/DR: even if a service provider is technically protected from the actions of their users it is still subject to provisions and conditions, and that still doesn't stop them from being sued and having to mount a defense. Some people just don't feel the hassle of all that justifies the whatever benefits they'd gain from fighting that fight.

Certainly you've heard of 'The Pirate Bay', who's 'users' famously used their platform to share copyrighted materials...the founders of The Pirate Bay were arrested, tried, and convicted, and were forced to serve jail time. Turns out the "but it was our user's doing it" defense wasn't as reliable as everyone here seems to be suggesting.