Earlier quoted context omitted.
You need to define a confidence interval for 'know'. We as a society judge and condemn people without 100% certainty as a matter of course. Both at a personal level and systematically, e.g. the judicial system. To your example of BMI, it would be a perfectly reasonable public health policy[0] to use machine learning to probabilistically identify persons of an excess BMI to send them pamplets and resources for weight…
I would not be a perfectly reasonable public health policy to do what you suggest, it would be a waste of time and money. Everybody knows that being fat is unhealthy. The people who are fat and not doing anything about it just don't care, or have other priorities.
But also, a few people are obese because they are on steroids or they have some genetic issue independent of their choices in life. They are not the norm, but they'll get lumped in with the norm and that's offensive and improper. That said, if FB found out their userbase is unusually obese compared to the rest of a country's citizens, that informs them they have a potential moral hazard in their hands.
But if you're deeply offended at such telemetry, maybe consider no using Facebook. You'll be fine without it. I personally make sure to post ridiculous and contradictory responses to the insipidly and horribly targeted ads they push into my feed. Some of them have gotten me suspended for violating their "community standards" that IMO would not get me suspended anywhere else.