They don't stop everybody, they stop, say, every third car. And they use high-pressure sales techniques to try to get "biological samples". But they actually don't arrest people they find impaired; they try to arrange transportation for them. And they don't claim to actually collect or register DNA, just the presence of drugs. I don't think that makes it right, but let's at least be accurate about what they're doing.
More information and links to past examples of these "studies":