@jonathanb
" If an agent of the copyright holder made the files available for download and advertised it on pirate sites, then the people who downloaded them obtained them legally."
You've raised a very interesting legal issue here.
Yes they did download them legally, but it's not the downloading the copyright mafia nail you for - it's the seeding / uploading, which is integral to bittorrent, and which legally amounts to unauthorised distribution. That's what they charge you with on the court documents.
I'd be very interested to see the legal take on this - if I, as a copyright owner, distribute my own work via bittorrent, I'm implicitly authorising others to distribute my work as well by the very act of using the bittorrent mechanism as my distribution medium. So does this implicit consent stand as legal authorisation for anyone in the swarm to distribute? What if I stop seeding the torrent myself - does that withdraw the consent for others to distribute? What if someone else creates another torrent on another tracker and starts seeding it there as well?
Bittorrent seems to be a real Pandora's box as far as legitimate or authorised distribution of copyrighted material is concerned. Any commentards here in the legal profession got a take on this, or does anyone know of any court cases dealing with these issues?