And whose idea of "best" do we use? The government is about ensuring a framework of freedom for its citizens to operate within, and if the citizenry refuses to be productive or healthy there's nothing the government can do about it.
Do you support alcohol prohibition? Alcohol leads to much more damage than even smoking from an emotional and psychological perspective.
What if, one day, the government decides that people who buy Japanese cars are a danger to themselves since they are costing the American car companies so much money by refusing to buy home-grown goods? The people just don't have the foresight to understand that buying Honda today means depression throughout Michigan tomorrow. Shouldn't we protect people from that eventuality by ensuring that they only buy domestic vehicles?
How far do we take this? Why do we believe that the politicians have such a heightened sense of foresight and such impeccably unselfish motives? I don't vote for people so they can do my thinking for me, I vote for people who I think will best represent important core principles, like retaining individual free exercise of conscience (like the choice to buy American or foreign cars, even if it will cost Michigan a lot of jobs).
The attitude that we need the government to protect us from our own decisions is in fact extremely dangerous and insidious. Old people survived for thousands of years before the implementation of social security -- if the industrial society doesn't support old people well enough, we have to tweak it so that these things work instead of doing these grotesque hacks of taking > 10% of an average person's salary every year to pay to support your grandpa's RV tour of the US.