vicarious has nothing to do with it...
Question: What is contributory infringement?
Answer: The other form of indirect infringement, contributory infringement, requires (1) knowledge of the infringing activity and (2) a material contribution -- actual assistance or inducement -- to the alleged piracy.
Posting access codes from authorized copies of software, serial numbers, or other tools to assist in accessing such software may subject you to liability. Providing a forum for uploading and downloading any copyrighted file or cracker utility may also be contributory infringement. Even though you may not actually make software directly available on your site, providing assistance (or supporting a forum in which others may provide assistance) in locating unauthorized copies of software, links to download sites, server space, or support for sites that do the above may contributorily infringe.
To succeed on a contributory infringement claim, the copyright owner must show that the webmaster or service provider actually knew or should have known of the infringing activity.
here is the problem.. they are technically not linking anything. they are providing meta data.. (suprnova that is). now the trackers could certainly be covered under this however this again isn't covered under a "forum" of any type, and they aren't directly transferring any copyrighted materials.
as I said, I'm not saying the law isn't being broken, but with bittorrent they are travelling into new and uncharted territory. There are many layers of accountability and unfortunately the only one that has legal precident (the users) are the exact ones the MPAA is wary to go after (don't punish the ones you need to make your money, a lesson learned by the RIAA).
the only recourse they truly have at this point would be to say the trackers are contributing, unfortunately today putting up a new tracker is pretty simple, not to mention doing it by way of IRC or IM'ing would make it very difficult to contain.
But I still believe that closing down suprnova or similar sites will require them to say that torrents or any sort of generated metadata is the same thing, or at least fascilitating, copyrighted material.
We'll see though. I hope they can contain it personally.. It is insanely out of hand. I just can't believe that if it were so easy and under existing laws that suprnova would have been able to be around as long as it has.