[Gianluca Roncaglia]

I believe it's permanently turned off (by the admin) during races for bandwidth savings. It has been left ON (by mistake) for a few races until it was discovered, then disappeared again. Maybe it causes glitches in races with a lot of drivers using it, the same way CM can cause freezes when updating new skins while driving.
RSR will stay off until you either start an offline race (that has penalties enabled), or edit the 'race.ini' file as mentioned above.

I think is a CM glitch anyway, with the standard AC launcher RSR works perfectly online. Not a major issue, but it would be nice having it fixed.