But once they step on US soil then the other laws kick in. They become US persons. So the challenge is keeping them out before they get here.
SCOTUS 17-965 Trump v. Hawaii June 26, 2018
“8USC 1182(f) and 1185(a) By its terms, §1182(f) exudes deference to the President in every clause. It entrusts to the President the decisions whether and when to suspend entry, whose entry to suspend, for how long, and on what conditions. It thus vests the President with “ample power” to impose entry restrictions in addition to those elsewhere enumerated in the INA.”
I believe the above text comes from the recent Supreme Court decision of the same date. It was posted upthread by another Freeper.
I believe the term, "US persons", specifically (and only) refers to legal US citizens, not aliens, of any legal status.
Merely setting foot on US soil does not automatically confer citizenship, or legal status. IF an alien steps onto US soil and requests asylum, then they are normally processed and given a citation to appear in court for an immigration hearing at a future date, then released on their own recognizance.
Only three percent of such aliens ever show up for their hearing. Ninety seven percent ignore the citation and melt away into the illegal alien population.