Once you've tracked the IP address to the account holder, how do you prove who the offender was in a household of like 10 people for example?
The point is, you can't hold the ISP account holder liable, unless you can prove beyond any doubt that the account holder was in fact the infringer, or was at the very least aware that the infringement was being committed.
At the end of the day, unless you're the original seeder of a particular file, I'd imagine these guys are going to have a hard time making an arrest. I'd imagine they'd have to fight dirty themselves in order to successfully achieve what they're setting out to do.
For example, I recall some legal entity claiming that you can't download a Torrent without partially uploading the file, which constitutes illegal distribution. Even if this was true, that would also mean that in order to collect IP addresses, you would have to download (and potentially upload) a portion of the file contained in the Torrent as well.
So the method used to obtain said IP addresses is most likely just as illegal as the actual leeching and seeding of the file in question.