In most countries you go through immigration not just when you arrive, but when you leave too.
In the US there are no ‘exit controls.’ When you go to the airport you get off a connecting flight and right onto your international flight, no more security and no immigration. Domestic and international flights leave from the same terminal.
The US is a real exception this way. Canada and the UK are also exceptions.
At least, the US was an exception.
Customs and Border Patrol documents leaked by the ACLU detail plans for new exit controls for the US.
In the coming months, walking across a US border will entail exit controls involving a passport scan (including for US citizens).
Random non-citizens are getting biometric scans on departure in Atlanta.
There are other experiments as well, like facial recognition for arriving US citizens to compare scans to passport photos.
The goal is to bring exit controls and facial recognition on arrival to US airports by 2020.