im sure the real reason other than being
ctp would be that the zune firmware uses a fair amount of the 16 mb ram, so i believe the xna team decided it was best to unload the majority of the firmware (apart from the nesecary system and xna software) so that developers can utilize the majority of the ram and that why the zune reboots after games to reload the unloaded firmware
Note just speculation on my part as i have no real proof since i dont know how much ram the zune actually has