Is it illegal? Probably; “it’s uuencoded chunks of unrelated textfiles” would get you laughed out of court for a newsgroup called “alt.stolen.games-and-mp3s”, which is what’s under discussion here.
It’s a civil matter, though, not criminal, so you’d have to get sued by the specific copyright holder. Which is really unlikely; tracking USENET is a bitch, unlike some of the P2P applications out there.
I’d say they’re not getting shut down because no one is profiting from them, other than the rather indirect ISP fee path. No rich targets to go after.