They certainly are using incentives to get you to share your personal XYZ to infinity so they can track you and build a file on you. Humana incentive program, for instance, wants to know where you shop, what you buy, your heart rate data, when you go to the gym, how many steps per day, etc, etc. The questions you ask certainly would be logical, slippery slope, conclusions at some point.
Not voluntary to the extent that you are paid incentives wrt kickbacks on insurance charged by your company and gift cards for the x amount of right things you do(go to gym x times gets x amount of gift card for instance), etc, etc. There is also the legit prevention aspects of people who exercise might not need as much healthcare dollars spent on them.
Having worked for a yuge analytics company they can store infinite amounts of data on you for a long time. Especially when it is the relatively few clients that an insurance company voluntarily gets permission to track as opposed to tracking all datapoints related to every visit to a website for x amount of clients that the big guys track.
Relating that data back to interesting things is being discussed ad-infinitum to the data holders/gatherers wrt to monetizing and/or reducing costs from theoretical conclusions derived from it. Shit is real. Not necessarily nefarious though. One day we might find out that is not the case. Is privacy an illusion?