The most obvious reason for the govt not to allow fat people to be charged 50% more is that they are about to start subsidizing insurance premiums. If Safeway's fat employees can't afford their premiums, they'll be allowed to purchase insurance on the exchange, with government subsidizing the cost of premiums above a certain amount, which is tied to income. Since lower-income people are more likely to be fat, this kind of fat penalty is going to cost the govt a lot of money.

There's also the fact that recent research shows that body weight is 70-80% genetically determined. The congress has just started passing laws forbidding genetic discrimination by insurance companies. Unless they want to backtrack on that, or start getting all unscientific about science again, they can't start discriminating against a problem with such a huge genetic component.