Here’s an interesting article on 4A and borders.
http://www.law2.byu.edu/jpl/papers/v19n2_Jon_Adams.pdf
The border exemption is based on the Constitutional power to ““to regulate Commerce with foreign Nations.” To do so requires prevention of smuggling and entry of unauthorized persons.
Funny, that clause is also used to regulate intrastate commerce [Wickard], and non-commerce [Raich]. — in fact, if the federal government tried to claim the same commerce-clause powers in foreign countries as they do in the states it would be regarded as an act of war... and the enforcement thereof would be either the waging of that war, or the post-war subjugation of the defeated. So then, it should be considered that the War on Drugs is waging war against the several states and therefore definitionally treason.