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.

I often fly from Milan Malpensa airport, and I’ve noticed there are two separate security areas: one for people flying to the US or Israel, and one for everyone else. I’d always wondered why this was the case, and now I get it.