A DUI checkpoint is a search of your personal property, without a warrant, and without cause. They set them up at random locations, and at random times. They stop every car that comes across, whether they were acting suspiciously or not, require everyone to open their window, whereupon they peer inside and look for anything that remotely resembles probable cause so that they can get grounds to 'search' you, when in most cases, they've already done exactly that.
More simply, a police officer can't stop you for no reason. They must have a reasonable belief that you've done something illegal.
A DUI checkpoint violates that first tenet straight off the bat.
Moreover, a police cannot search you without probable cause, but requiring you to open your window while they shine a flashlight throughout your car, after already having stopped you for no reason, is going even further.
I won't bother touching upon situations where dogs are trained to 'alert' for illegal substances whether or not they exist (because I honestly don't know the frequency), but that also happens.
I'm not a lawyer and therefore not qualified to expound on what "Three-pronged Test" or "Doctrine" decides what qualifies as an invasion of privacy, but...
If the cops could easily tell you were driving drunk, they'd pull you over and cuff you. They have no particular reason to suspect you're DUI if they have an armed checkpoint set up. There's the first problem, "no particular reason". That's not supposed to happen in America, that was what happened in Nazi Germany, and Soviet Russia.
The second problem comes from what's probably the real reason, or at least an important subsidiary reason, for the "DUI" armed checkpoints. According to some "N-Pronged Test" or "Doctrine" that non-lawyers can't possibly understand, the inside of your car is "public". If a cop sees something fishy, he or she can just search your car. That's the second problem: "DUI" checkpoints rapidly become fishing expeditions for whatever minor illegalities the cops can find.
The penalty structure for any crime or administrative infraction (at least in the USA) is set pretty high. This may have made sense when cops were few and far between. The USA had to make penalties pretty severe so that folks would stop at stop signs even when a cop wasn't there to watch. You didn't get caught very often, even if you did whatever crime. A fairly harsh punishment made sense in sparse cop circumstances.
But once you universally enforce some "sparse cop/harsh penalty" punishment regime, you've gone way off the ranch, and you're probably just doing it to increase revenue, either through fines, or through privatized prisons that need their bunks overfilled.
Many states have now set up DUI Checkpoints. These are not targeted based on behavior: Every car traveling between point A and point B is expected to pull over and take a breathalyzer. These are often targeted based on neighborhoods or proximity to 'party' districts. They are conducted without warrants, and cast a wide net on individuals traveling.
I see. Okay, I didn't know that happened. I've only ran into one such checkpoint in my life and the policeman just asked me where I was coming from and where I was going, asked if I had anything to drink while pointing his flashlight into my car; I assume looking for an open can/bottle or anything suspicious. That last like 4 seconds, then I was on my way.
I'm going to guess you're white and probably look reasonably mainstream. Somebody more "suspicious"-looking will often end up with a less smooth experience.
I think I'd like the DUI comparison to be expanded on a bit. How are they being used to invade one's privacy without grounds?