Any specific infringement material (by which I mean media) would only be on the user’s home server. Links to content aren’t what is actionable for a DMCA notice as far as I’m aware. And the DMCA does not require platforms to actively monitor or remove potentially infringing content, only to follow the takedown procedure when sent an appropriate notification. If they follow that then they are protected from liability. That’s US law but IIRC the implementations in most of the rest of the world are similar if not the same. And here’s the rub: even without those communities, LW will still need to have a DMCA agent and take action against content when notified because people can and will upload infringing media here on other communities.
They’re not exposing themselves to additional risk by having the piracy communities unblocked. People can and will discuss piracy, in abstract terms at the very least, all over the place. And discussion of copyright infringement is not copyright infringement anyway. Any liability and risk they do hold they will still have to worry about now regardless.