How would you effectively stop windows updates to china? Bypassing the Windows activation measures isn't stopping single pirates (or people still wanting Windows 7 updates intended for big business with legacy devices), the only way to stop windows updates is preventing access to it. It's impossible to do so while still having Windows as an Operating System that people outside high security environments use.

To be fair, Microsoft could be persuaded to add actual exploits to Windows contingent on the install being in China. In that case, they would want the Windows updates to be installed. There is thus no realistic scenario where exports of Windows updates would be possible or even desired by the US government.