A related story that hasn't received much traction on HN: >US wants to use facial recognition on air travelers leaving the country https://qz.com/1598148/us-wants-to-use-facial-recognition-on...
The US is kind of unique in that it doesn't have immigration officers who stamp your passport when you leave ... so you have no proof that you have left ... it also means that there are no transit lounges in the US, you can't land in SFO from say Australia and get on a plane to London without passing through US customs and spending time and money getting a US visa (even an ESTA takes time and money)
At the moment the US depends on airlines to get it right, it used to be that when you arrived they stapled a green bit of cardboard into your passport, sometimes the airlines would forget to collect it as you left, or lose it and people would end up getting arrested when they come back (a night or two in the cells and thrown on a plane back home). For a while there were kiosks in the departure lounge. Now days it's done electronically, and they still get it wrong occasionally, the error rate however is probably way below 3% at the moment.
The horror stories from people caught this way mean that people from Oz/NZ often prefer to travel to Europe the other way around rather than risking US travel