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It is more an issue of immigration control than anything else. The USA currently does not have government-operated exit immigration control checkpoints at large international airports. Instead, airlines are in charge of swiping travel documents and uploading the APIS data to CBP/ICE to determine an individual's permission to leave the country.

The issue for duty free is that it means there is no sterile international departures area ("point of no return") for departing international pax. In most of the world, as a departing international passenger, once you have cleared exit immigration and security, you cannot just waltz out the door and reenter the country. If you changed your mind about traveling, you would have to be escorted back into the entry immigration area and be processed as an arriving traveler. The USA does not operate that way. It doesn't matter if you are flying to Cleveland or Frankfurt; if you wish, you simply can walk out of the airport at any point prior to boarding. And that creates a problem for duty/tax free sales, because there is no way to be 100% sure that those items, particularly alcohol and tobacco, will not be walked out of the secured area and used inside the USA without paying federal duty and any state and local taxes as applicable.

So, really, the only way for duty free businesses to remain in compliance with the law is for them to deliver restricted bonded merchandise directly to the aircraft door, which is the last point at which the traveler no longer has the option to waltz out the door and import those goods illegally.

Make sense?