AFAICT, the line is 100 miles from the border as defined as "the edge of the US," not as defined as "100 miles from a foreign country."

This line[1] encompasses places even as far inland as Columbus, Ohio, and Columbus, Georgia, which are good examples of being perhaps least border-feeling, non-coastal cities that I've ever spent time in.

I wouldn't blame anyone for having never visited the grand metropolis of Fort Wayne, Indiana, but that's within 100 miles of the line, too.

Lots of other unlikely cities are this way are this way as well.

[1]: Here's a map on ArcGIS. I only found it; I did not have any part in creating it: https://www.arcgis.com/apps/mapviewer/index.html?webmap=f43e...