Society does have a reponsibility to not provide those substances that cause serious physical harm to many people more readily and cheaply than those more suited to good life.

Yet we don't tax sugar, processed food is the cheapest and non-processed are the most expensive.

The entire food industry is designed to sell more of 'their' product, and the addictiveness is developed more than the health value. Lobbyists ensure the governments keep the status quo.

You can surely make people fat, or lead-poisoned, or stuffed full of sugar and diabetic, or more full of meat, etc. through government decisions.

While the libertarians might have a point that most such decisions turn out to be deleterious, it's certainly not the case that policy can't make you fat, and so can presumably avoid making you fatter.

We managed to decrease smoking rates by 50%. It seems reasonable to assume that reducing obesity rates is a fairly similar problem.

It's intriguing how morale is again on the stage; we went from "obesity is caused by lack of will" to "it's not deserving we pay for that research".

Isn't that likely, though, that this research pays for itself?

How does research pay for itself? Naturally the results of the research could well be worth the time and money invested, even many times over if you're lucky, but that doesn't mean that it paid for itself. Somebody still has to pony up the dough to pay the scientists, lab assistants, equipment, time for them to figure out what they're doing, etc. None of it is free.