If you're using BitTorrent they can just record all the IPs that are currently active on that torrent. Anyone can see who's connected to the torrent, it doesn't take much to record all the IPs and then contact the ISPs. You don't have to upload anything to be visible on the torrent.
USENET is completely different though, a lot of News servers don't keep logs and if the ISP doesn't log the data either there's virtually no chance of getting caught, as far as I'm aware.