Yes, I live in the zone, and could be subject to searches. In fact, I am subject to such a zone every time enter or leave the security of my place of work. But these "border searches" are taking place far from any real border, on public roads, and are practiced on the general public.
I appreciate that the ACLU is resisting these types of actions, but I wanted to understand from where exactly this authority derives. After a little research, I found some answers. The authority comes in two parts, one from the law, and another from administrative regulation. The law is 8 USC 1357, whose crucial part reads,
1357 (a) Powers without warrant
Any [authorized] officer ... shall have power without warrant—...
(3) within a reasonable distance from any external boundary of the United States, to board and search for aliens any vessel within the territorial waters of the United States and any railway car, aircraft, conveyance, or vehicle
So there you have it, within a "reasonable distance," border agents may search any vehicle without a warrant. But what distance is reasonable? Well, that's where we need to turn to the administrative regulation, as encoded in the Code of Federal Regulations (8 CFR 287.1 specifically), which specifies,
(2) Reasonable distance. The term reasonable distance, as used in section 287(a) (3) of the Act, means within 100 air miles from any external boundary of the United States or any shorter distance ...
These regulations are written by the executive agencies, as a way of codifying and interpreting how the law will actually be implemented. In this case, 100 miles is seen as "reasonable" by definition, rather than by any other claimed rationale.
But is this anything new? No, in fact. The definition of a 100-mile wide border has existed for at least several decades, and does not begin with the current administration. I found that the 100-mile limit was specified in the CFR as early as 1953 (Fernandez v. United States; 321 F.2d 283). The appeals court in that case found that a search 70 miles from the Mexican border was reasonable -- given the specific geographical and immigration situation. As of the date of that case in 1963, the "border" checkpoint 70 miles from Mexico had been in operation for 31 years, so this practice has existed for at least 75 years!
However, it is clear that the practice of unwarranted border searches is expanding, and it's a troubling sign. The truly scary part is that the search for illegal immigrants is being used as a pretext for other searches, such as for contraband. For law enforcement, it's a no-brainer. Why send "regular" police to do inspections if they need complicated stuff like warrants, when it's easier to send "border protection agents," who don't really need anything to start tearing through your personal belongings. As various agencies, including the border patrol, immigration and customs have been consolidated into the Department of Homeland Security, it is even easier to designate an everyday enforcement activity to be a warrantless border patrol activity. As more "border crossing" inspection sites are instituted, the more more we become a police society.
As a postscript, there are some daring people challenging new inspection sites. Consider Mr. checkpointusa.org, who has been documenting the increase of the interior "border" checkpoints in the southwestern U.S. for the past few years. You can see some of his encounters here and href. He routinely approaches the checkpoints and takes control of the situation. Dude has balls of granite.