FXDoggie,
A US company could do what they like, allow or disallow. He's employed by a US company in the US, what his obligation is to another country is of no concern to his US employer. This FA18 pilot was employed by a sub of CX, based in the US and therefore US Law applies and if CX don't like that they have the right to close up shop and move their employees to HK or elsewhere.
If a foreign tax service had the required muscle to enforce that I'm pretty sure the US company would comply.