Never understood that part. Do they check the tracker or just send warnings based on p2p network usage? For example, there are many legal things you can torrent. But you're right, it's more of a hassle to check for illegal streams.
Also I'm wrong about what I said in previous post, because there is DMCA... not that it matters much in todays polarized world.
If I'm downloading a game of thrones torrent, someone affiliated with HBO will be tracking IP addresses of those who are downloading or uploading the content, and then they send cease and desist letters to that ISP who owns the IP address. If you get two (possibly three) warnings regarding this, they shut down your internet. And since there's only one cable provider for anywhere you live in USA (the cable companies divvy up locations), if you get your cable cut off you have some shitty choices to find another ISP.