1) They have to figure out who the people actually are, either through adept mass-Googling of the usernames or getting IPs that the users log in from.
2) There's only a small portion of the music that they can figure out is illegal from this--stuff they played before it was released. Even though they like trying to charge 10x for such songs in lawsuits, 1 or 2 albums won't be worth much.
3) I don't think that that's sufficient evidence to win a lawsuit (though maybe sufficient evidence for them to bully people into settlements as they often do.) I could have found the track list for an upcoming album and tagged any songs I have with the artist and titles of those songs.
Still, CBS definitely shouldn't have done that.