[schoen]

Yes, this is the case for airports that offer "sterile transit" (making an international connection without formally arriving in the transit country). In that case, international passengers and domestic passengers would be segregate (with Schengen passengers treated as "domestic" in the Schengen area).

The U.S. doesn't offer this, seemingly as a matter of policy, something which really annoys international travelers -- it's one of the most-complained-about U.S. policies on sites like FlyerTalk, even by U.S. citizens (because you can easily spend 1-2 hours clearing immigration, customs, and security for an international flight connection in the U.S. even if you didn't intend to visit the U.S. at all).

But as a result, almost all U.S. airports have some departing international and domestic passengers in the same terminal at the same time, or potentially allow both international and domestic departures from any terminal.

[mentat]

Houston used to do this in 2000. Don't know what has happened since. (Flew Mexico to Japan without entering US.)

[schoen]

I also heard that LAX used to have sterile transit for a particular pair of flights (I think operated by the same airline).

[mikeash]

I imagine it might come down to how much money the airline is willing to put into the necessary infrastructure. Paranoid security changes after 2001 might also have shut that sort of thing down.

[schoen]

Yeah, maybe an airline can say to the airport/CBP "we'll pay to rent a lounge/corridor/gates at this airport in this terminal for a sterile transit option between our flights or our partners' flights because some passengers want that" and maybe the airport/CBP can approve the proposal. (Maybe non-U.S. passport holders will still require an ESTA, but not a CBP examination?)

I think the specific option at LAX was post-2001, but you had to be on a specific pair of flights to take advantage of it. (I didn't do it, I just read about it on FlyerTalk and forgot the details.)