You missed my point. First of all, ALL radio is technically supposed to pay mechanical royalties to artists
I think we've got some mixed-up terms here, and I don't know if it's you, or the rest of the Internet. As I understand it, mechanical royalties are paid for the duplication of stuff that stores music (whether vinyl, CDs, or mp3s), and thus, have nothing to do with radio. What you seem to be talking about is what the Internet calls "performance royalties".
These are now broken down into two separate categories, old-school "performance royalties", and "digital performance royalties". With the former, only the original songwriter gets paid for radio play, while with the latter, both the songwriter and the recording artist get paid. The former is what terrestrial radio pays, and the latter is what satellite radio and Pandora pay. In the metal world this is probably irrelevant even if metal *was* big on commercial radio, because the songwriters and recording artists are usually the same people.
I just wanted to point out that Sirius is a special case, and in terms of royalty payments, is treated more like Internet radio than terrestrial radio (because many people may be surprised to learn that in the last 100 years conventional radio has paid less money specifically to recording artists than Spotify has paid them in the last week).
But yeah, Sirius, terrestrial radio, and Pandora are all "non-interactive" services, while Spotify, iTunes, and Rhapsody are "interactive" (on-demand) services, so that's another way to classify them.
Neil