The US does not have exit immigration. Airlines have to ask for your passport to ensure that you meet the entry requirements of your destination country. If an airline transports a passenger who is then denied entry, that airline will be fined and on the hook for getting the passenger out again. Generally they can recoup these fines from the passenger but it's a pain and they don't want to deal with it.
The EU and Japan, for instance, do have exit checks. These are done by the relevant immigration bureaus. The passport checks performed by airline personnel are completely different and bear no impact on one's ability to leave the US.
As an aside, this is the reason that most international connection at US airports require a visa. US airports (with LAX being the sole exception, AFAIK) have no way to monitor anyone leaving the secure area. Nothing would stop passengers in transit between international flights from simply leaving. Connecting in, say, Tokyo-Narita doesn't require entering Japan because everyone in the secure area has either exited Japan or is in transit in the sterile zone.
The only exception is passengers on Air New Zealand's Auckland-LAX-London run and Air France's Paris-LAX-Tahiti flight, though I may be wrong. Special arrangements have been set up for these flights.
The EU and Japan, for instance, do have exit checks.
Not if you leave from an airport in the Schengen area to another Schengen airport.
Some airports may check your ID (and determine if it matches your boarding pass) before allowing you into the restricted, passenger only area. But others don't. In Vienna and Zurich you just automatically scan the bar code on the boarding pass and an automatic gate admits you. Some of the gates now even have automated gates where you scan your boarding pass and get admitted to the plane.
There is also no entrance check, when you arrive from a Schengen airport.
I usually don't need an id when flying within Schengen (Prague being one of the exceptions), just the boarding pass, which I usually print at home.
Please note that I don't recommend not to carry id. Even if you're usually not asked. If they do check at the gate you're quite obviously not boarding when you don't have a valid id.
Of course, excuse my Amero-centrism. I should've written that the Schengen Zone does not have exit checks, which is what I meant when I mistakenly wrote "EU."
I miss my time in Europe. Flying around Schengen nations is so much easier than between states these days.
That's not entirely accurate. Since the arrival of the US-VISIT program, visa holders need to record their departure. At least at some locations and times, this was done with a mobile "pseudo-checkpoint" near the gate area. You weren't required to talk to them, though.
The EU and Japan, for instance, do have exit checks. These are done by the relevant immigration bureaus. The passport checks performed by airline personnel are completely different and bear no impact on one's ability to leave the US.
As an aside, this is the reason that most international connection at US airports require a visa. US airports (with LAX being the sole exception, AFAIK) have no way to monitor anyone leaving the secure area. Nothing would stop passengers in transit between international flights from simply leaving. Connecting in, say, Tokyo-Narita doesn't require entering Japan because everyone in the secure area has either exited Japan or is in transit in the sterile zone.
The only exception is passengers on Air New Zealand's Auckland-LAX-London run and Air France's Paris-LAX-Tahiti flight, though I may be wrong. Special arrangements have been set up for these flights.