Sure, requiring companies to remake all their packaging and redesign ads will be expensive... for the companies. Instead of forcing the people who suffer from the effects of obesity to shoulder the financial burden, placing the responsibility on the causation side may help light a fire under the collective butts of those who produce unhealthy products.
Of course, there will always be those who say that measures like San Francisco's are classic, unnecessary government interference with individual freedom. But, knowing what we know, and considering all the cancer cases we've avoided treating, does anyone blink an eye at cigarette warnings now?
Reducing sugar consumption is still an uphill battle
Americans down a lot of sugar, mostly from sweetened beverages -- the average adult consumes 20tsp a day. Compare that to what the American Heart Association recommends: 6tsp (around 24g) for women and 9tsp(around 36g) for men. Considering a can of Coke has 39g of sugar, it's easy to blow right past the daily recommended intake with just one drink. Obvious warning labels could help people rethink their drink choices, leading to a calorie deficit of at least a couple hundred a day.