Fred Hiatt at the Washington Post has an interesting article on obesity and the government. The answer to his question and title to his article is fairly simple. It became a partisan issue once people began insinuating the government should interfere.
Yes, obesity is on the rise. I don't know if anyone actually questions that (disagreeing with this policy position does not mean one denies the factual rise of obesity). The questions we should really be asking is 1) what is causing this rise, 2) what should we do about it, and 3) who is this "we?" That last question will determine how question 2 is answered and is really the linchpin to the whole argument.
Question 1 is fairly complex and Mr. Hiatt completely fails to answer it. Bad eating habits are hardly new in America. Something that is fairly new over the last thirty years is the sedentary lifestyle. Computers and video games within the home have largely replaced outdoor activity. This sedentary style itself encourages bad eating habits (it is easier to down a coke and Cheetos while watching the screen than it is while running around outside). I'm not going to claim to know exactly why obesity is on the rise, but I do want to point out that the businesses that make sugar products and hamburgers are not solely responsible.
Question 2 depends on the answer to Question 3. This is where the issue becomes partisan. If "we" means the government, than the answer is taxing sugar goods and banning fast food restaurants from operating in certain areas. Mr. Hiatt implies that Mrs. Obama was not pushing such solutions but rather a voluntary reduction; problem is, the government has already begun adopting these nanny state solutions. NY was considering a soda tax and San Francisco has banned toys from happy meals.
Conservatives and libertarians answer question 2 differently. The government has no authority or responsibility to fight obesity; it is largely a personal choice to be made by individuals. There is an important philosophical difference here. For statists, the government knows best what is good for us. The responsibility for decisions, then, passes from the individual to the government. People who support the policies tend to believe their value judgements are best, to the point where everyone else should have to follow them at the point of the gun, if necessary. That is, after all, what laws are.
On the other hand, those pushing for less government believe people know best. They hold the pluralist view that there are different forms of good that are not always compatible with each other. It may very well be that for person A, obesity is acceptable so long as they can enjoy their sedentary lifestyle and sugary goods. If the government passes laws to change person A's habits, they are actually making person A less happy overall. With 300,000,000 citizens having different preference structures, it is impossible for the government to know what will make them all happy. Any attempt to force those people into any one policy for their own good will inevitably force many people into less happiness.
Would forcing people to exercise and eat better lead to better health? Probably. But will that lead people to be happier than they otherwise would have been? No, in many cases. Those who believe health is more important are free to do so, but they need to stop and realize others may (rightly) disagree based on their personal preferences. If you are passionate about healthy living, make your best case and convince as many others as you can to change their ways, but leave the taxes and the force of law out of it.