Originally Posted by stifle

The reason for this is that most US airports allow you to simply walk out of the airside area at any time through an exit door or passage. So a person with an international boarding pass could buy duty free and then not fly (or buy it, take it back to their car, go through security again, and fly). In Europe and elsewhere, it is mainly the case that there is no way to walk out of the departures area. You usually need to be escorted, or pass customs controls, or both.