China is using North Korea as part of it's leveraging for Taiwain. It's a big political game, and fortunately we aren't doing the same Korean blackmail song and dance we did in 1994 that proved fruitless and expensive.
Korea is more dependent on China than many think. If China were to pull out aid, North Korea would implode. It's why we have the 6 way talks, and why we don't bilaterally tallk to North Korea. That and Kim cannot be trusted to hold to any agreement, as he's proven time and again.