As Taniwha pointed out above[0]:

> If you make that argument then the customs officers are also NOT on US soil and have no authority to search phones (or do anything) at all

[0] https://news.ycombinator.com/item?id=21518813

Even if that's true, that simply means the search moves a few feet to US soil, where they stand on one side of the border and you stand on the other. And they don't have to let you in if you're not a US citizen or lawful resident.

Doesn't being subject to search apply regardless of citizenship status?

No, if you're a citizen or lawful resident they have to let you in...eventually. Courts have ruled that they can hold you at the border for a few hours.

and constitutionally speaking, permanent exile isn't considered a punishment for noncitizens, so SCOTUS can rule whatever they want is unconstitutional, it still means they're gonna do it