Lemmy.world maintains a local copy of every external community. This is how federation works. Any piracy related posts on those subs will be copied in their entirety to lemmy.world servers, so lemmy.world could potentially be sued for hosting that content. Being the largest instance makes it a target.
It is rare to get advanced notice of legal problems. Usually the first you hear about it is a cease and desist, or a lawsuit. Lawsuits are costly to defend even if you’re doing nothing wrong.
I don’t like this decision. But it is a sensible one to protect the instance. If you care about piracy discussions you can visit those communities directly or on a different instance that made a different decision.