Google isn't hosting the files that point to trackers and ultimately other peers to download from.
There are cases similar to this: Galoob v Nintendo. In this case Galoob was cleared of charges as it was found that users of Game Genie, while altering games, were not creating derivative works that violated Copyright.
Kelly v. Arriba Soft Corporation Professional photographer vs image search service. It was decided since Arriba was just showing thumbnails and did not have the actual files on their servers they were in the clear.
I personally think the charges are a little out of hand. That said if you don't have rights to the files you are doing something wrong. The repercussions of the pirate bay winning or losing this case are definitely going to be interesting.