Right, most are for ad space on the outside of the busses. And why not? In America wealthy people drive their own cars along side busses, so that makes the most sense.
I'm not judging, heck I'd like to see America better utilize mass transit, but for marketers it's all about best bang for your buck. In China that exposure is inside the bus. If that ever changes here in the States, we'll see an American version of CCME pop up and I'll stick a nice chunk of cash in it early on! Heck, maybe CCME will go multi-national :)