Not advertising, monetization
That is the real problem. If they can integrate the information into the rest of your profile, that provides a lot of information that can be monetized, for example selling information to health insurance companies etc.
This person does ~3,000 steps a day, has a generally high heart rate and searches a lot for near-by branches of McDonald's and Burger King, searches for Doritos and Nacho Cheese dips and streams hours of video every day from YouTube... If FitBit starts also doing blood pressure, blood oxygen levels etc. That provides a lot of additional pointers. Put in extra information from things like App usage, uses Uber Eats every day, uses an electro-roller (instead of walking or using a push bike) etc. and you have a lot of very "bad" information for the FitBit data slave, sorry, FitBit users.