I suppose another geopolitical aspect of this is that China doesn't want to be dependent on other nations for fuel. If they became involved in a war or even just anger their trading partners in some way, the rest of the world can shut off their energy supply.

For that matter, even if they're on great terms with all their trading partners, those trading partners might ask themselves "why are we selling coal to some other country so they can get the benefit when we have to live with the CO2 emissions as if we had burned it ourselves?" To be honest, I don't understand why the U.S. exports coal at all; it just doesn't seem worth it. (Though I can see where the coal lobby might have something to say about it if such legislation was proposed.)