fwiw, i started dating a lawyer and she and I went deep into the 100-mile free zone topic. Me, on the paranoid side and she, siding with the SCOTUS. We spent a few hours researching Supreme Court cases and eventually found lots of details. I'll try to relate the most important nuances. 1. it's not an official u.s. policy. i love the ACLU, i'm a member, but they're inducing some unnecessary fear. 2. you can't be unreasonably searched, they have to have some reason. typically, this reason is immigration. 3. this power to search has been challenged in the Supreme Court repeatedly. in some cases it was upheld, b/c the primary reason for the search was immigration. in others, the executive branch could not hold up the case and it was dropped. so, checks and balances still work. 4. after searching for immigration, anything they find such as drugs or explosives is utterly admissible. so immigration is often used as a pretense, but they can't get away with arbitrary immigration searches 5. it's not just 100 miles around the border, it's also around every port of entry, such as airports.

I hope this helps. I really wish she or other lawyers like her would interpret this stuff for everyday people like you and me. The government and the ACLU both seem to benefit from our ignorance (and the fear we inherit due to it), so neither is incentivized to present the true picture.. and that's the real problem.