How well do these trackers prevent - rather than invite - legal prosecution for their users?
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.
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.