The argument I make is that the Constitution exists to simultaneously establish a government and limit its powers, and applies to all agents of said government no matter where in the universe they may be.
If the border crossing is not within the jurisdiction of the US, its agents have no authority there, and if it is within the jurisdiction of the US, its agents are subject to its laws and bound to obey them. And if the agent stands one foot in and one foot out, the person seeking entry ought to be able to turn around and return whence they came with all their possessions intact whenever they decide the crossing has become too burdensome and that they no longer wish to enter.
A citizen of France has as much right to be free of unreasonable search and seizure by an agent of the US government in New York as in Paris, or in London. The Constitution is the adhesion contract for all persons seeking to become employees of the nation. It just so happens that no court in the US has been established asserting jurisdiction over territory outside the US, so if said Frenchperson had experienced a violation of their 4th Amendment rights by, for instance, a CIA spook without a warrant, in their own home in France, there is no court available to take the case other than the Supreme Court itself. It simply won't take the case, just because no one wants to open that can of worms.
The de facto situation is that the US government does whatever TF it wants outside the US, because it has the money, might, and influence to get away with it, and the voting public back home just can't give enough focus to international shenanigans as well as their regular, day-to-day problems.
By a strict reading, it breaks its own laws all the time, with absolute impunity. As a lifelong American, I have never heard any politician so much as mention this, even to dismiss it out of hand as a non-issue.
Unlocking an electronic device to examine the bits stored on it is, in my opinion, an unreasonable search and seizure that goes well beyond the scope of the eminently reasonable searches for and seizures of physical contraband at border crossings. Therefore, by its own rules for unreasonable searches, a warrant must be obtained by the US government from itself before said unlocking can happen.
Furthermore, if I saw a sign saying, "YOU HAVE NO RIGHTS", I would be pissed. Regardless of any laws in force, everyone has the right to be treated with a default level of respect and basic human dignity by their fellow humans, and to travel about their own planet without some arrogant yard ape turning their trip into a GDMF Kafka novella. You have the right to be addressed courteously. You have the right to protest any government overreach, even if it trivial, and even if it is not your government. You have the right to live free in the free world.