When public health care costs rise due to obesity, diabetes, smoking, and the like, it is often said that an externality exists,justifying public intervention. The logic is that as costs to Medicare and Medicaid rise, so too must taxes to offset the higher costs. Thus, my health care costs (if I'm enrolled in Medicare or Medicaid) impose an externality on you the taxpayer.
I've written several times in the past suggesting that this sort of argument is not particularly well founded (e.g., see here or here or here). I ran across another line of reasoning in a post about immigration and the welfare state by Don Boudreaux that suggests how slippery a slope this sort of reasoning can be.
The logic used to assert that existence of public health care benefits justifies restricting (or altering) consumers' choice of foods is no different than the logic Boudreaux uses (in jest) to argue that the existence of welfare programs and public unemployment benefits justifies restricting (or altering) student's choice of college majors.