China may not be our enemy (yet) but they are definitely America's enemy, and America lets us play with their nice toys (AMRAAM, F35 etc.). They also invite us to share intelligence and take part in their programmes, RAF pilots are currently flying B2, F35 and F22 in America, would they let us do this if there was a risk these pilots would subsequently go and fly for China? I have spoken to a number of US exchange officers about this and they are absolutely furious, don't imagine the Pentagon is overly impressed either. It may not be technically illegal but neither is having sex with a 12 year old in the Philippines. Ultimately your actions in teaching the Chinese how to be better could end up being used to kill Western or Taiwanese pilots, if you can live with that, fine. I couldn't.