but they literally do seed torrents to get a list of all clients who connect to them and download

Actually, if I recall correctly they tend to seed only partial files because what they actually want to hit people with is distribution. If I understand the legal situation around it correctly, the act of downloading is much harder to pursue in civil court as opposed to damages for distribution. Because they need to prove that you're not just downloading it for yourself but distributing it to others, hence sitting in the swarm and logging IPs of anyone who sends data to them.