So asks Anne Applebaum at Slate:

For not only is China the country with the most influence over North Korea, China also is, along with South Korea and Japan, one of the countries most under threat from North Korean nukes. After all, it is China, not the United States, that will be at the center of the new Asian arms race if Japan and South Korea feel compelled to get the bomb. It is China, not the United States, that would feel the effects of fallout if North Korea actually used its weapons. Although it isn't clear whether North Korean missiles can reach Hawaii, it's obvious that Beijing, Shanghai, and Hong Kong are well within range. So, shouldn't this be China's problem, not ours?

Haha, quite right. Well, part of it is that they're not really worried about a nuclear attack from North Korea. They're allies. But the biggest thing is this: They want us to deal with it. As long as the US is preoccupied with North Korea's nuclear drama, we're spending less time criticizing China's human rights (which are bad, but aren't really in the same league as NK's), less time/energy/money spying on them, and so forth. And North Korea keeps demanding to deal directly with us because they know a unilateral agreement is about as binding as a piece of toilet paper.

So it's a win/win for the Chinese. Even know NK should really be their problem, strategically it makes sense for them not to touch it with a ten-foot pole, but to dump it in our lap and let us spend our time and effort on the problem.

Doesn't that make their action, or in this case inaction, pretty shitty? Well yeah. Wake the hell up...China isn't doing anybody any favors. The US may be criticized around the world for sticking its nose where it doesn't belong, usually out of self-interest, but not always. But countries like Russia and China not only do nothing to help with military, humanitarian crises around the world, or with nuclear proliferation...they actually exacerbate the problem by supporting some of the most noxious regimes around.

In the case of North Korea, they may actually help out a little, but they're damn sure not going to take a leading role. They're going to sit on their hands at let us do it. Because they know we will.