Drinking is a liberty, driving is a liberty. Drinking and then driving is not a liberty, it is against the law. There is no oppressed freedom to drink and drive.
You're not providing probable cause when you turn away, but you are providing reasonable suspicion that "criminal activity is afoot." I don't fault you for wanting to avoid the checkpoint, you just have to do it the right way so your freedom isn't temporarily restricted by the local constabulary. And it won't be the officers working the checkpoint that are watching for you to turn around, it will be the officers assigned to watch for people turning around and heading the other way.
On a related note: When a checkpoint is set up the coordinator decides how many vehicles are going to be stopped at one time. While the checkpoint is full of vehicles, all other traffic will be allowed to pass until it is time for more vehicles to enter. Did you know that EVERY vehicle selected has to enter the checkpoint? This includes all emergency vehicles (police, fire, ambulance), even if they are on duty, unless they are on an emergency run.
Despite what some lawyers and politicians would have you believe, the Constitution was written in plain English, not "legalese" that needs to be "interpreted."
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.
Please tell me how a "sobriety checkpoint" doesn't violate the security of my person. If someone is driving in an unsafe or erratic manner, that's one thing (PC, in fact). But to detain people just so you can "test" them for sobriety (and how can any such test not be considered a form of search)? It takes a lot of twisting, not to mention gall, to try to twist that to not being a violation of the fourth.