The Skippers discretion is absolute.
You can quote whatever regs you like but if the Captain is not happy then the flight does not move or the pax/bags are removed. That is the case law the world over.
Unless you can prove a malicious intent by the Skipper you will lose every time.
Jurisdiction does not really come into it as in reality the Skipper can close up, push, move under his own power, become God and order whatever he or she wishes.
The USSS are not making a big deal of this. They are probably embarassed.
Just imagine IF this guy had been accepted and it turned out to be a terrorist. The crew would have been castigated as grossly negligent.
If you have ever been in the LHS under pressure you would have a whole lot more respect for the AA Skipper who made this decision.
Armchair internet wannabe Skippers need not comment.
PS