They could sue every single private server out there now...if they wanted.
The reason that they don't do it, is because no server is making that much money or has so many players to make it even worth sueing them.
Once NTreev sees a server as a threat to their playerbase, they will probably sue them.
Just look at WoW servers and Blizzard. Do they sue servers? Yes, but only those that are worth sueing.