China is a supporter of the Iranian regime and has continued to do trade and business and investments in Iran, despite the fact that Iran is on the far end of the political ostracized spectrum.
China is starting to shake things up and agitate Japan and Taiwan, with the whole sea control thing and the islands.
Add to that the whole currency scandal that has been following China for the past years... and the human rights violations...
So there are already 4 black stains on its world reputation. Now, to add to that we have China being pretty much the only supporter of NK. What is the difference between NK, Iran, Japan and Taiwan? NK is the only nation that is very dependent on China. Iran doesn't need China more than China needs Iran and Japan, Taiwan and other nations that are at odds with China are well developed nations on which China has no dirt on. They can't force those countries to do anything because they are independent on China and have bigger and better friends (USA, the EU).
My guess is that China doesn't want to spread its political capital too thin. It wants to maintain a good relationship with power players around the world and a healthy image in the world. Having a stupid, loud neighbor that you're tied to in the public image hurts that. And if you can get him to be quiet... you may just earn the gratitude of many. So it pays off for China to put pressure on the young prick to shut up and take the credit on the public arena for that.