Ragnor: Is NZ Post guilty of assisting copyright infringement if UserA sends a pirated copy of Sione's Wedding to UserB in the mail?
If you asked most people they would say no, if you apply some basic common sense you would probably conclude it's not NZ Post's fault the service was misused/abused.
Now the torrent site situation is similar but not the same, they enable User to many Users publically, not just User to User privately. Saying it's 100% the Users doing the dirty is not going to work as a defense for torrent trackers.
Or just use Google to find the stuff you want. Is Google guilty as well?