Apart from the sovereignty issues, you need a whole load of American expats from the Customs & Border Protection to man the desks - chatting to them last time it sounded like quite a nice assignment for a few years! For 2 flights a day it isn't going to be worth it. Why they can't do it at Heathrow, where there is the critical scale, is another issue (I guess the flights to the US all going from different terminals wouldn't help as a starting point!).