The issue with the USA's southern border is the potential of uncontrolled access to the USA from every country in Central and South America, and by extension every other country in the world. (Some of those countries now have very lax "visitor" entry control.) Even just considering Central and South America there have been a number very violent crimes (rape, murder, armed robbery) perpetrated by non-Mexican illegals that entered the country by crossing that Mexican/US border undetected. It seems that while Mexico won't let those interlopers stay in Mexico (they actually enforce their immigration laws, with a vengeance) they don't much care if someone is just "passing through". Fortunately our neighbors to the north do have serious entry control very similar to that of the USA, and no other bordering countries to deal with.
And that entry control is the "wall" for air traffic (and for seafaring traffic) which makes your aircraft argument pure BS. Those aircraft that you refer to can't land just anywhere, gotta be an airport, and fly through "controlled" airspace with a flight plan, to get to one. Aircraft that have had an International stop must first land at an airport that has a Customs presence, which includes that Passport Control. And EVERYONE on the flight has to go through Passport Control, even the President. (Yes, they have Customs with Passport Control at Joint Base Andrews). Part of that includes verifying that the traveler has the proper immunizations if they're traveling to or from most countries (some, like Western European countries, are exempt from that requirement, but passport control WILL scan your VISA stamps in your passport to see if there's any stamps from non-exempt locations and there WILL be more questions if there are and the traveler lacks that current official immunization paperwork. And while the required vaccinations will vary by destination, all will require the useless TB vaccine (which does work great for leprosy) along with Hepatitis A and B.
This post was edited by my2cents at November 3, 2017 5:40 PM MDT