Medicare and Medicaid as justification for public health interventions

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.

And, while we’re at it, doesn’t the existence of the welfare state require government also to restrict which majors college students choose? Without a welfare state, students would be more focused on finding gainful employment after they graduate. But with a welfare state, the risk of being unemployed for long periods – or of earning very low pay for most of one’s working life – as a result of majoring in the likes of “race studies” or “dance criticism” will too often be ignored by irresponsible or lazy students, who rely upon welfare-state payments to subsidize their indulgence in majors that promise no decent monetary rewards.

Where does the enhanced scope for government action end once we admit that government buys for itself, by illegitimately exercising power W, an indulgence for the exercise of otherwise illegitimate power R? What sort of distrust of the motives and knowledge of government officials leads many self-described libertarians to oppose government’s exercise of power W but approve of government’s exercise of otherwise-illegitimate power R if government insists on simultaneously exercising illegitimate power W?

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.