Basically like firing you from work for discrimination isn’t legal, but firing you for performance is (your performance being anything they wish to come up with).

It has been ruled by SCOTUS that the Constitution specifically does not apply at border entry to non-citizens:

https://en.wikipedia.org/wiki/United_States_ex_rel._Knauff_v._Shaughnessy

the Constitution does not grant aliens any protections when trying to enter the United States.

https://constitution.congress.gov/browse/essay/artI-S8-C18-8-7-3/ALDE_00001263/[‘article’, ‘14’]

an alien “on the threshold of initial entry stands on a different footing” because he or she is theoretically outside the United States and typically beyond the veil of constitutional protection.

https://supreme.justia.com/cases/federal/us/345/206/

Although a lawfully resident alien may not captiously be deprived of his constitutional rights [to due process], the alien in this case is an entrant alien or “assimilated to that status” for constitutional purposes.

The Attorney General therefore may exclude this alien without a hearing, as authorized by the emergency regulations promulgated pursuant to the Passport Act, and need not disclose the evidence upon which that determination rests.