U.S. customs agents are searching more cellphones

The premise is that it’s not unreasonable to consider smuggling as a common behavior at border crossings, and civilian citizens are only protected from an unreasonable search by the fourth amendment.

They can hold you for long periods of time, irradiate you with x-rays and inspect bodily cavities with gloved fingers, or have medical personnel do it for them, and that’s all considered “reasonable” given the context of an international boundary.

Airports at which customs agents search entrants to the USA are, in fact, in the USA and US laws absolutely do apply there. But there are lots of conditions, including search incident to border crossing, where warrants are not required for a search to be reasonable (and thus legal) in US law.