Not sure you are correct on the first point. Border Patrol are law enforcement officers. Though they are primarily focused on border enforcement, I don't believe they are confined to that. A Border Patrol agent could probably give you a speeding ticket regardless of whether you had recently crossed the border, if he observed you speeding (guessing few would do so however).
If you're saying that absent any other reason, you would have to have recently crossed a border for the initial stop to be legal, maybe you're right. But random checkpoints for intoxicated drivers happen everywhere, and are legal.
> If you're saying that absent any other reason, you would have to have recently crossed a border for the initial stop to be legal, maybe you're right. But random checkpoints for intoxicated drivers happen everywhere, and are legal.
As you point out, Border Patrol are fully sworn law enforcement officers and can therefore enforce other laws besides mere border protection statutes.
However the legal requirements to perform a search of a vehicle (incl. the need to have crossed the border) are different from the requirements to stop a vehicle to interview the driver (which any LE can do incl. Border Patrol), which is one of the reasons that DUI checkpoints are legal in some circumstances.
If you're saying that absent any other reason, you would have to have recently crossed a border for the initial stop to be legal, maybe you're right. But random checkpoints for intoxicated drivers happen everywhere, and are legal.