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.