You’re under that impression because the ACLU did some very dishonest reporting about that subject. (I’m an ACLU member, it’s a fantastic organization. But it’s marketing people have a questionable commitment to educating the public truthfully.)

The “100 mile” regulation involves the scope of CBP patrol authority: https://constitutioncenter.org/blog/does-a-constitution-free....

The law, 8 USC 1357, says that CBP agents have the power to patrol and search “a reasonable distance from the border” to prevent entry of illegal aliens.

First, calling that a “Constitution-free zone” is rhetoric, not fact. 8 USC 1357 only refers to the CBP’s power to patrol and search, not other Constitutional protections. And while there can be reasonable disagreement about where the line should be, clearly there must be some distance within the US where the CBP must be able to patrol and search without a warrant. The border is an infinitely thin line. CBP can’t patrol foreign territory. It necessarily must patrol US territory some distance inward from the actual border.

The “100 mile” regulation was an upper limit on what “reasonable distance” could be. The actual regulation requires the CBP to define “reasonable distance” based on geography, where border crossings are, etc. The idea is that checkpoints must be at the “functional equivalent of the border.” In the middle of the desert or the Alaska tundra, the checkpoint might reasonably not be exactly at the geographic border. But in no event can it be more than 100 miles from the geographic border.

So all those graphics of how many people live within 100 miles of the border were very misleading. New York City is not within the extended border, even though it’s within 100 miles of the border, and CBP has never claimed otherwise.