For a high-IQ society, the Chinese oddly tend to get stuck in these narrow mental ruts. For years, China's foreign ministry has spent most of its time cracking the whip on much smaller, typically third-world countries when the latter step out of line. Withdrawing pandas from, say, the Brunei Zoo is a non-destructive way to assert dominance, and works because the Sultan cares more about showing off his pandas than he does about tariffs on Chinese noodles or whatever.
In dealing with the U.S., China's most important tool is PR. The whole point is for Joe Sixpack to associate China with cuddly pandas, rather than with his last bout of unemployment caused by cancelled orders for whiskey or soybeans. So as silly as this kind of thing is, they're shooting themselves in the foot.