> as long as you are within a radius of 100 miles from the border.

The rule is more nuanced than that. Until you cross all border control points then you can still be searches as if you were crossing a border. The rule you're talking about is that these border control points can be within 100 miles from the border to allow for efficiency of operation of border controls. Imagine if literally every border crossing to Canada had to have a dedicated CBP outpost manned every hour that the road itself is open.