North Korea is useful to China. They're a rogue state that China has influence over. It gives China a certain amount of diplomatic leverage. "Well, you could try some economic action against us... But we're all that's keeping those darned North Koreans in line. Are you sure..?" But there does come a point where North Korea becomes a liability. At that point, China won't hesitate to step in.
Actually, they might just step aside and let us deal with it. They wouldn't like having us increase our influence in the area, but they might like it more than they would having to actually deal with North Korea's lack of infrastructure. Whoever ends up having to deal with North Korea, it's going to be expensive.