Ummmm, probably because he took part in actions that killed Americans. That's like whining about how the triggerman gets more time than the guy who rented the getaway car.
But in the giving spirit of this holiday season, I suggest President Bush cut his sentence in half, and have him serve the rest in a military brig. Among the servicemen there. In general population. With inattentive guards.
Working KP as a dishwasher. Next to the cooks. Next to the cooks with knives.