Obesity has a cause, and people have a responsibility to manage their own health.

But does it really make sense to say obesity is someone's 'fault'? Choices have consequences, but fault requires unambiguous harm.

I disagree, or rather i think you're too absolute. My cousin was obese at 8 (after a car accident this parents started to eat a lot, and as the youngest he followed). He had prediabetes at 12 and that killed his hormonal balance. He now cannot be anything else than obese, even with calorie restrictions.

Your cousin must not have read as much Ayn Rand as OP.