It is a published fact known around the world that China's Air Force controls the airspaces in China and they only release about 20-30% of airspace to commercial traffic. It has nothing to do with how high they can fly, or how low they can fly, it's about whether they have the green light to fly at all.

Also, China's Air Force is known to hold last minute "drills" and will close down an air space at a second's notice. Many of my colleagues have told me their flights were turned back MID-FLIGHT because the air space is closed off for military exercises.