Setting the “100 mile” thingamajig aside - why wouldn’t you consider an international airport part of your border? It is a port of entry.

The problem is the 100 mile rule, which effectively puts most of the population under the purview of border control measures.

Apart from that, an airport is a comparatively small area of land that is magnitudes easier to monitor than a border that only exists as a thousands of miles long line on the map, therefore the 100 miles rule should not be applied. And everyone getting off an international flight in the USA has already been searched multiple times.