There is no 100-mile border zone. That's a marketing creation of the ACLU. This question was put to the Supreme Court in the 1970s, and the Supreme Court unequivocally held otherwise.
The nexus with reality of that "100 mile" figure is that it represents the limit of the distance at which the USG can establish a permanent checkpoint --- at which CBP is allowed only to stop people who have actually crossed the border. Some DHS person misspoke when describing that process, and ACLU picked it up as "the USG believes it has border search rights for anyone living within 100 miles of the border". Which is a farcical position.
The nexus with reality of that "100 mile" figure is that it represents the limit of the distance at which the USG can establish a permanent checkpoint --- at which CBP is allowed only to stop people who have actually crossed the border. Some DHS person misspoke when describing that process, and ACLU picked it up as "the USG believes it has border search rights for anyone living within 100 miles of the border". Which is a farcical position.