It's cheaper than dropping the price of the console by 50 bucks (360 at $350 might sell another 1.5 million 360's but would cost them = $75) I think the lower price (than ps3) + GTA shared content + GTA exclusive content ($50 mil) will sell atleast 2 million systems (earning the company 50 million more than if it had dropped the price).
I would cite regulation, but I know you will simply ignore it.