> I don't see where OP is implying that individuals suffering from unfortunate accidents outside of their control are driving up health care costs
No, OP is just proposing that help is denied to everyone who needs it and can't afford it, because it is deemed more important to prevent free-riders than to be humane. It's a nasty ideology, that is only embraced by the US among all developed countries.
It means that your life is only valuable to society if you have (or had) some economic value. It doesn't matter, by the way, if this economic value comes from actually contributing, or by inheritance, or by winning big at a casino. The important thing is that you have money, otherwise go (literally) die in a corner.
> This whole discussion seems to be centered on lifestyle choices, eating large quantities of sugar, that have a significant effect on the health care costs of the population as a whole.
So why not tax the big corps that make all of this junk for the costs to society that they externalize? Sure, it would be great if people had the level of education necessary to make the right choices. It is pretty hard to get that level of education in the US if you are born into an unprivileged situation, because the education system for the poor is shit, because said companies avoid paying taxes as much as possible, and the more well-off are selfish and shortsighted, and vote for politicians that do not spend public money on what can improve society as a whole.
> Childhood cancer or other unfortunate illnesses are statistically too infrequent to be driving up health care costs alone.
You are very young, I imagine. Perhaps you haven't yet started to understand that things go to shit as you age, and it is not "infrequent", it is part of the human condition. A part that is hidden out of the public eye because it is inconvenient to talk about it. All Americans are immortal and about to be millionaires!