Probably don't want to piss off Japanese developers. If they pulled out of Japan probably some Japanese developers would threaten to not make games on the 360 anymore/360 successors.
Worst case scenario as Salnax said; you want to be in that market if the shit hits the fans with either Nintendo or Sony, because while they are a 3rd choice in Japan they could b the 2nd choice if someone decides to leave the market.
It's just that simple.