"MS isn't gaining much in this"
Uh...Rock Band has brought in roughly $275,000,000 for EA from the 360 version alone - that's the rough equivilent of a 5m seller. MS certainly has enough power, and reason, to get EA to do it (and a guy named Peter Moore).
Every other version of RB hasn't done stellar, unlike the 360. With the fact that the 360's version has a proven track record, I am sure that MS gave EA a cut somewhere - DLC fees (of which, the 360 version has probably made $20m or so from the DLC alone), accessory costs (MS could be making $20 from each unit alone), or something else.
Either way, it's a smart move for MS. The music market for games is huge, and Rock Band is obviously the best player right now.
Back from the dead, I'm afraid.