I would prefer that China plays a more active role in resolving the situation so I sort of hate to play devil's advocate here but...

The missile defense systems we deployed in SK could be used to track Chinese missiles. My point was imagine the outcry from the US if we perceived that the Chinese were tracking our missiles under the guise of tracking Mexico's.

Their problem isn't with the interceptors; it's with the tracking radars. They could theoretically help counteract Chinese missiles and aircraft that are supposed to deny access to Chinese coastal waters in the case of war with the US.