The US requirement is that passengers on flights to the USA have been processed in conformance with US regulations _and_ since that processing have not had any contact with passengers processed otherwise. It's not in itself a stupid rule but does make the US rules contagious, since either other airports re-build to keep the US-bound and other passengers segregated or they have to apply US rules to all.
This hit Auckland International badly: it had a lovely open atrium with a garden but the rules forced a forest of partitioning walls since passengers were transferring from smaller airports that couldn't quickly adopt the US rules.
alt Hacker News
The US requirement is that passengers on flights to the USA have been processed in conformance with US regulations _and_ since that processing have not had any contact with passengers processed otherwise. It's not in itself a stupid rule but does make the US rules contagious, since either other airports re-build to keep the US-bound and other passengers segregated or they have to apply US rules to all.
This hit Auckland International badly: it had a lovely open atrium with a garden but the rules forced a forest of partitioning walls since passengers were transferring from smaller airports that couldn't quickly adopt the US rules.