Is the way our culture beats people with the healthy stick really about [an entirely demented concept of] what's good for you? Or is it about conformity?It's definitely about conformity, but even more than that, it's about control, and moral judgement of others.
Even though this is supposedly a scientific information age, people still feel on some level that being not healthy means you did something wrong and you're being punished for it.
Health is in fact mostly a matter of luck (chance, genes, environment). One can have some influence on one's health conditions through behavior and environment, but one cannot absolutely control them and one cannot pick which health problems one is going to have to deal with. But people desperately want to believe that their health is entirely in their control, and part of sustaining that myth is to look down on people who are farther away from the health norm than they are, and believe "they did it to themselves." The other part is to look at their own health status, largely influenced by chance, and believe "I made this, I am this healthy entirely because of my own choices."
People do the same sort of thing with poverty. Even though there are enormous social and economic forces keeping poor people poor and rich people rich, people look at poor people and want to believe "They're there because they're lazy." And people look at themselves, if they aren't poor, and want to believe "I am a self-made success through hard work and sacrifice."
Note: I see this has come out implying that everybody always thinks this way. I don't really think so. But I do think these are general trends and attitudes that are part of the social fabric, and everybody who is part of the social fabric is influenced in some way by these beliefs.