Now I ask anybody to explain to me how the U.S. Constitution protects foreigners seeking to enter this country for anything?
Foreigners do not, in fact, enjoy full Constitutional protection.
Though they are permitted the protection of due process, none has a right to a visa or any form of a grant of entry.
None.
Also, the default is not that a failure to observe due process, by U.S. government agents, results in "automatic" entry. Rather, the default is, that if entry is denied and the applicant desires to pursue the matter in court, the applicant still does not gain entrance --- and instead, they are represented in absentia; though a legislating - judge may decide to order an entrance, that judge would be extraConstitutional, the order, invalid.