This isn't just a Chinese thing. I've experienced it personally in Russia, and I've heard of it in other countries too.
I call bullshit. You get fined almost always for driving unbuckled. It's actually one of the better effects of bad and greedy traffic police, who will otherwise fine you at the first available possibility (and, sigh, oftentimes impose bribe on you).
And here are similar experiences to mine: http://www.passportmagazine.ru/article/786/ http://www.pistonheads.com/gassing/topic.asp?h=0&f=23...?
Rear seat passengers do not wear seat belts indeed, but that's another story: lots of cars simply don't have them and it's not enforced by law.