First of all, what? It is the job of the government, again, to protect the well-being and security of the people. They are defenders first and foremost. We aren't asking elected officials to somehow intrinsically know what's best, we're asking them to, when an issue arises, take all of the facts into account and come to a decision, tempered by what they know are the desires of their constituency, that is for the betterment of:
A ) the people they will directly effect, and
B ) the future generations that their decisions will more than likely effect.
There is a direct link between unhealthy eating and obesity, heart disease, increased cholesterol and blood pressure, etc. The issue of public health has come up. It is the government's job to take everything into account and make a judgment call about what is the best course of action to take to protect the people (in this case, from bad health). These particular politicians have done just that.
The ultimate goal of any organism (in the micro- or macrocosm) is to survive. If people are dying from something, it is the job of the government to curtail that because if all of our citizens die, so does our country. It's why, even if free market demands power plants to be near a residential area, the government has deemed it necessary to, through zoning, prevent power plants from being built near homes (health being one of the reasons). It's not like this isn't without some sort of precedent.