Well, it’s considered “okay” because the search is invisible/not physically invasive or disruptive and none of the data gathered against citizens via these means is admissible in court (at least in theory).
I do think there’s a much more serious argument that what citizens are subjected to at airports and at the border is a violation of the 4th though.
For those unfamiliar, the US fourth amendment text:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.