FòrumCAT

It's generally seen as okay on a similar level to undercover work. They do it for Investigation reasons, the torrent was already uploaded before they joined, their monitoring serves a legitimate law enforcement purpose, and they're authorized by the copyright holder (themselves) to do it. They didn't put the movie or whatever out there themselves.

Their methods are fine, they literally just pirate the stuff themselves, see which IPs connect to them, then connect those to an ISP and notify them. The main reasons you wouldn't get notices are getting lucky, not seeding much, not torrenting things that are being monitored, or having an ISP that doesn't care much.

The single notice from the streaming site makes sense, pirate streaming sites are usually honeypots or heavily monitored.