There is economic externality for the healthcare system in the future for excessive consumption of bad diets. If we want to reduce punishment to other taxpayers, it might be rational to nudge people towards healthier behaviors.

Such policy might include tax breaks for exercises, taxes on cigarettes, soda, etc. Tax subsidies for veggies, etc. Obviously solid evidence is needed before it is initiated.

There could be philosophical arguments that the government should not interfere with their citizens' lives. But this is more about charging the true (future) costs of actions, like carbon tax, rather than dictating behaviors.