Your argument, and you see this nihilistic tendency often in privacy discussions, is also based on some strange notion that if your privacy has been violated once (and you didn't voice your disapproval), it may be acceptably violated again. I see no reason for this to be.
I'm not arguing that it's how it should be. I'm arguing that it's the way it is now in many parts of the world.
It would be a normal procedure to request the car's owner to disable the dashcam before the service, and not to agree to service the car with the dashcam on.
But no dealer has the right to turn off the dashcam himself.
But dealership is not a public place.
Additionally you are only allowed to use cameras for security purposes if you can make a case that there is no other way to protect yourself. And before you're in view of any camera it must have been stated explicitly, or the cameras must be obvious before you're being filmed.