Possibly also used by the natives themselves. PX stands for Post Exchange which is basically the military department store. It sold goods that the average local store didn't have. Servicemen would bring their native girlfriends or wives shopping on base. As they understood it, America had all of these goods and more. It was their main association with the US.
I can't wait to get out of this shithole country and go back to the Land of the Big PX.