Frankly, I think that we should have national health insurance, paid through our taxes, but that isn't a popular position these days.
Given that it is so hard to maintain weight loss, the number of factors that go into weight gain, including genetic factors, my opinion is that a punitive charge is not going to help.
Here is an interesting article on the issue of charging people different rates, depending on whether they are obese, or smokers, or the like:
http://www.nytimes.com/2011/11/17/he...pagewanted=all