What I heard was that TM Revolution got asked to do anime songs
because his popularity was dropping and he wasn't seen as some big-artist-who-could-not-be-tarnished-with-a-tie-up... yet he would be profitable enough for the anime because he was well-known anyway.
Possibly could be the same scenario with Hyde (Blood+) and L'arc~en~ciel (FMA)
On an unrelated note I found out some time ago something interesting - now I know why in Rurouni Kenshin they had changed the ED to Fourth Avenue Cafe by Laraku, but after a short time they suddenly changed the song back to Heart of Sword, the previous ED, but still with this new ED's visuals. They took the song out because one of the band members at the time was arrested for a drug-related offense and their songs were taken out from everywhere, including anime.
I think singers like TMR will still get their revenue from singing mostly, I don't think he's been "condemned" to do only anime songs from now on... he's presumedly got all that money from his previous successes too
.