Ah, it's Thomas Ptacek, who as usual, doesn't have the slightest fucking idea of what he's talking about but nevertheless feels free to lecture others.
There are dozens of immigration/law enforcement checkpoints routinely deployed around the U.S. They stop all persons and feel free to check them for suspected illegal immigration, drugs, and so on, with absolutely no reason to believe the searchees have ever crossed the border.
For instance, the Border Patrol runs a check near Camp Pendleton, California, about 70 miles north of the Mexican border. You can even check in with Foursquare:
https://foursquare.com/v/i5-border-patrol-checkpoint--weigh-...
All cars on the interstate are stopped, any time they feel like it, and searched or not as the Border Patrol desires. No suspicion exists or need exist. Most of those stopped have not crossed the Mexican border in years, or ever. And no, none of it is illegal in any way.
Or compare, if you like, the EXIT searches that Customs is now doing at many border crossings - people who are attempting to (but have not yet) exited the United States. By definition, they aren't entering the U.S. and haven't crossed any border, but again, they're being searched, entirely legally, every day, and woe to you if you attempt to resist, as author (and now convicted felon) Peter Watts will be happy to tell you.
Simple rule of thumb: if tptacek says it, you can ignore it.