It is one of the angles in dealing with China. What would China get in return for going along with U.S. regarding N. Korea? A puppet regime in N. Korea and the conquest of Taiwan, along with no trade retaliation? Or does China have to choose between Taiwan and N. Korea again? Certainly China wants to have the former, but it would be asking too much.
True, they are trying the salami tactics: First get international OK to monitor, border and inspect KOR vessels. then...