The BitTorrent tracker contains a list of IPs of people connected to each individual torrent. Everyone that connects to the tracker obtains the list for that torrent so that you can find other peers with pieces of the file. All they have to do is connect to the tracker and record the IPs. Then they file Joe Doe lawsuits to find out who those IPs belong to.
{update} I assume that they have to wait for a connection from someone to exchange a piece of the movie though. It is possible to connect to a tracker, falsely advertise the percentage of the file that you have, and just sit there disallowing connections. Unless you transfer something other than communication w/ the tracker I don't know how they could find that you've infringed copyright. [Maybe someone should purposely do this to try and get sued by them.]
Is it illegal to set up a shop that purports to sell illegal devices, even though you don't actually have any illegal devices and never buy or sell an illegal device?
AFAIK, you can't download a torrent without participating in seeding it (at least while you're downloading it). Assuming that whomever is gathering the IP addresses is an authorized representative of the copyright holder, they are actively involved in distributing the video (at least while they're collecting IP addresses).
So, a defendant could claim "At the time you caught me downloading it, an authorized representative of the copyright holder was publishing the content on BitTorrent".
Of course, the problem is that the law isn't logical ;)
> So, a defendant could claim "At the time you caught me downloading it, an authorized representative of the copyright holder was publishing the content on BitTorrent".
They could be publishing data, but you'd have to prove that they weren't just uploading bogus data into the stream. BitTorrent hashes all of the chunks it downloads, but that doesn't prevent someone from constantly uploading bogus data that is only rejected once you've downloaded it. [IIRC, there are clients that will blacklist peers that consistently send corrupt chunks though.] They could just claim that they were uploading random data, and you would have to have some way to refute that (which you wouldn't unless you keep an audit trail of all peers you connect to over BitTorrent).
Most bittorrent clients rank their peers by upload speed when they choose who to upload their pieces to. You could still download a file if you refused to upload (afaik seeds will upload to anyone), but you would probably experience a decrease in speed.
Apart from the fact that you can disallow download and only query the tracker (you'd get horrible download speeds but presumably that wouldn't matter for this use case), you logic is flawed. Merely co-distributing pieces of material doesn't automatically grant others use rights. The co-sharing could be a necessary step in discovering the identity of infringers.
{update} I assume that they have to wait for a connection from someone to exchange a piece of the movie though. It is possible to connect to a tracker, falsely advertise the percentage of the file that you have, and just sit there disallowing connections. Unless you transfer something other than communication w/ the tracker I don't know how they could find that you've infringed copyright. [Maybe someone should purposely do this to try and get sued by them.]
Is it illegal to set up a shop that purports to sell illegal devices, even though you don't actually have any illegal devices and never buy or sell an illegal device?