Probably because they're not done suing 11 year old girls for not buying enough CDs. Or they just haven't gotten around to figuring out exactly what claim they want to file.
The last frame of the strip was the one I was after (the bit about humming the tune), but the back story starts
here (and runs to the strip I highlighted -- Sundays aren't part of the usual story flow, BTW), and perhaps is more relevant.
(in any case, it's amusing
)
Elsewhere, the same strip suggests that the RIAA would like to broaden the definition of piracy to include getting a song "stuck in your head."