Such systems are indeed voluntary - my company provides communications network services for 4 car companies for this type of service and other (ala OnStar) drive assistance purposes.
However, people also ought to remember that cars have a 30 second "black box" data recorder that monitors a ton of stuff inside the vehicle
without the owner's explicit "opt-in" permissions. This unit is in all modern cars due to governmental requirements.
That black box data
has been used for law enforcement and insurance companies to determine "at fault" situations in accidents, speeding situations, etc. That is how they can often say that "excessive speed was the cause of an accident" for example.
Also, FWIW, the OBD-II device mileage reports (called "Usage Based Insurance" or UBI) has been proven to reduce insurance costs. [For the moment anyway,] driving habits are not monitored ... only the number of miles driven is used to calculate the insurance costs and it does help (see Metromile at
http://www.metromile.com - one of our customers - for an example). The Progressive product is also one of these units (less of an upstart though, MoD) and there are others in the works.
We are a monitored society ... don't
ever forget that!
Z