It is a private tracker, but anyone can sign up for the site and use it, so whether anyone will look is questionable - I mean, the big American players like the MPAA and RIAA prooobably don't care about us swapping JPop stuff. If UFA or the AKB folks get in on it, it's possible they could start looking into it. I'm not SUPER worried, but you never know.
I got a warning like that from my ISP once a few years ago after downloading an episode of Shugo Chara from a public tracker - I think this was right around when Crunchyroll (or whoever, I don't remember) had licensed it.
It's up to the site admins re: whether they want to crank up the security on the tracker, I guess.