I am a US citizen. Once on a visit to Basel I walked without a passport to France and then to Germany, and then back to Switzerland.
No one stopped me. Thanks be to the Schengen Agreement.
Friends of mine, in the glory days of the 1990s, went canoeing in the Boundary Waters. They went to the Canada side and 'claimed' some island for the US in jest.
They didn't get stopped, much less detained for two weeks.
In the same glory days, people on the Maine/Canada border would frequently pass back and forth without a problem. Eg, to get gas in the US, where it was cheaper.
That's why there are still things like https://en.wikipedia.org/wiki/Haskell_Free_Library_and_Opera... . Which, you'll notice, allows people from Canada to enter the door on the US side without going through customs.
So it is not unreasonable that there can be flexibility on what it means to cross the border.