Because it's pertinent to the discussion:
Amendment IV
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
IMO, the constitutionality of these checkpoints is subjective. Each person will form their own opinion based on their interpretation of the language used in the 4th amendment. What constitutes an "unreasonable search"?
Legally, this practice has been defined by federal courts as constitutional. That's a fact. Whether or not this definition is correct is up to each of us to decide for ourselves.
For me, I find it comparable to this scenario:
I am sitting at home on a Sunday afternoon, fully obeying the law, and minding my own business. A police officer on foot strolls up to my front door and rings the bell. He informs me that he and his partner are searching for a fugitive who they believe is hiding in my neighborhood. I inform him that I have not seen their suspect, nor is the suspect in my house. He proceeds to walk around my residence, shining his flashlight inside windows and peeking through blinds to ensure that what I have told him is truthful and correct. The officer never enters my residence, and satisfied that I am being truthful and finding no probable cause to enter my residence, continues on his way.
As the officer leaves, I imagine I would find myself somewhat irritated that he did not take my word for it and move on. I would wonder why he chose to look around, and would feel to a degree that my privacy was violated.
Although the officer never entered my residence, thus never requiring probable cause or a warrant, in essence a search was still conducted. The same applies to DWI checkpoints. They shine their flashlights in the back seat, look over your vehicle for other violations, and generally look for an excuse to charge you, even if you are a perfectly law abiding citizen.
I don't take a major issue with it though, because like I said, I think it is one of the few DWI prevention measures that actually have a measurable effect on the problem, and I've got more important and pressing things to worry about.