The difference here is that dying from cancer because you won't take chemo is not contagious.I have said many times society has the right to make decisions for children that it does not have for adults.They are still abusing their children and killing themselves and making us all foot the bill for it as they do it.
I think vegans are slowly killing themselves but I don't tell people what to eat.
And the person who turns down chemo does not tie up the healthcare system denying access for the rest of us.
(And turning down chemo is often a rational choice. It can come down to a tradeoff between quality of life and quantity of life.)