Why don't US airports have customs and border protection inspections?
Congress has given executive branch of the federal government the authority to impose inspects on exit. And in fact these do happen on an ad hoc basis. Ive been inspected in the jetway of a flight leaving the USA at least once. Mostly the CBP officers that day were looking for passengers sneaking $10,000.01 or more out of the country without declaring it. The reason why these inspections dont happen on a normal basis is cost. CBP can get most of the benefits of exit controls by asking the TSA to refer to it any findings of currency in excess of $10,000 and requiring airlines to submit the passenger manifest (including passport details) to CBP before the aircraft departs. That all said, I think it would be in the economic advantage of the USAs international airports and airlines if airports had Asian/European style transit zones where passengers arriving from international flights dont go through CBP inspection unless they leave the transit zone. Many more passengers would transit the USA: Asia/Europe -> Canada/Mexico/Central America/South America/Caribbean via the USA and Mexico/Central America/South America/Caribbean -> Canada/Asia/Europe via the USA. My belief is that the CBP and Dallas Fort Worth (DFW) airport will convert Terminal D into the USAs first international transit zone. DFW is ideal for this first trial: Terminals A through E are physically isolated from each other. There is plenty of space to add CBP check points before the escalators and elevators that take passengers to platform of the inter-terminal passenger train stations. DFW is a convenient gateway to the Caribbean, Mexico, Central America, and Latin America. DFW Terminal D has lots of airport lounges that international transit travelers crave. Terminal D is spacious enough to add many more stores. DFW has 7 runways, 4 of which are over 13,000 feet, and plenty long for a fully loaded wide body aircraft to land and liftoff. International airlines based outside the USA would be begging to operate connecting service at DFW, and this would be a huge boon for travelers to and from the Western Hemisphere. British Airways for example could fly A380s between Londons two runway airport and DFW, and then use 787s and even 737s to connect passengers to the rest of the Western Hemisphere.