Hmm,
Well how about that fact that the US has sought to get other nations to bear part of the cost of developing the JSF in order to make the plane affordable for the US? The US needs arms exports as much as other nations need to buy arms. In real terms a Spitfire cost c. £100,000 to build in WW2, the Typhoon now costs c. £75,000,00 each, how long before even the US can not afford to develop an aircraft on it’s own?
Also bear in mind the political advantages that the US gets by having the UK along for the ride in it’s military adventures.
It’s not that the arms trade goes just one way. Remember that all US navy pilots are now trained in a UK plane (Hawk) and that the US president will soon be flown around in a UK/Italian helicopter (Merlin) and that the US Marines currently use a UK fighter (Harrier).