Because most games don't have to do with shooting up completely innocent and random teenagers, be it in the US, China, or the Middle East.
I have yet to see a "Super Madrasah Shoot-up" yes, COD4 involves a war in the Middle East with strong parellels to the war the US cooked up, but you'll notice the point of the game isn't to shoot up kids.
EDITED IN HOURS AFTER THE FACT:
Thus, this is not a case of Western hypocrisy. It's perfectly reasonable to be offended by Columbine games.
Columbine High School BTW, is full of rich people, in a rich little segment of Denver (called Littleton) in case anyone was curious.
If that sort of thing had happened at Montbello High, in the armpit of the 'burbs out here no one would have heard about it; a student gets killed every year at that school, starting in 1980, so that's 28 students.