The US is not the be all and end all of an airline's operations. As was shown on 11/09 and also the SARS virus that having all your eggs in small basket's is quite a bad thing especially for a small airline - hence the need to expand into other markets and offer a variety of desinations to as may different corners of the globe as possible, so if there is a problem in one part, people will still travel to the other parts and keep you in business.
Just take a look at most of the US carriers with restricted global operations, most of them are struggling to stay alive.
The US is no longer the place to be in global commercial aviation, there are much better more profitable places to fly to.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"