> The federal government defines a “reasonable distance” as 100 air miles from any external boundary of the U.S. So, combining this federal regulation and the federal law regarding warrantless vehicle searches, CBP claims authority to board a bus or train without a warrant anywhere within this 100-mile zone. Two-thirds of the U.S. population, or about 200 million people, reside within this expanded border region, according to the 2010 census. Most of the 10 largest cities in the U.S., such as New York City, Los Angeles, and Chicago, fall in this region. Some states, like Florida, lie entirely within this border band so their entire populations are impacted.

If you check this in the search bar at the bottom of the page you'll see this has come up a bunch on HN (it's a lurid and exciting notion to toss around!) and there are SCOTUS cites in the results.

It is not my contention that CBP isn't abusing the law. I'm sure they are. Many CBP immigration stops apprehend no or single-digit unlawful immigrants while arresting dozens and dozens of citizens for things like suspected drug offenses. My contention --- well, not really mine so much as that of the Supreme Court and of law review articles about this case law --- is that what they're doing is plainly unlawful.