The people that go after these sites are more interested in prosecuting the site operators than the users. I think they want to make running a private torrent site such a risky endeavour that no one wants to take the risk.
That said the best sites have operators with some experience at avoiding the law.
they don't really do much of anything to prevent legal prosecution. they're only still around because they're relatively small, invite-only communities. if it ever went public it would get shut down in a day.
Its also pretty pointless when the Mafiaa do manage to get a site taken down new ones crop up almost immediately. What.cd was taken down recently and now has 2 or 3 viable replacements.