When it comes down to it, Pirate Bay is simply a tool. Its not like the 4 on trial went to movies or bought games, ripped them, then distributed them on the internet. They created a tool to find .torrents. By US law they would have to prove that the owners of pirate bay partook in distribution and theft. This is not what they were on trial for in Sweden. When you put it in this term it would set a bad precidence to charge someone with a crime in this circumstance.
They could easily convolute it to prosecute pry-bar manufacturers because their tool was used in theft. Or charge convenience store owners because they allowed a robbery to happen on their premise.
This is why ISPs don't worry about RIAA demanding them to monitor users, they cannot hold them accountable in court.