Having lived near the US border most my life, the way it was explained to me years ago, is that there is a strip of land at the border crossing, where you are in neither country. In this space, you are not privy to any of the protections you would be allowed were you in either the US or Canada. This is where you are taken when you are forced to leave your car. Part of the current controversy is that the US government is trying to treat this literal no man’s land as both international land as as federal land with the disparate charges they are applying.

Fun fact. Lack of parking is not a legitimate reason to attempt to drive past this region. I was once flagged crossing into canada on a day they were interviewing a lot of travelers. I spotted parking in what looked to me like an overflow lot on the other side of a fence, and when I slowly drove that direction sirens blasted and guns were drawn. Nothing gives you clarity like a gun pointed at you.