Good points. Though it's actually not a zero sum game, it's worse than that. When you get down to it there's a very important link between the satiety/calorie ratio of food and weight gain. Sure you can talk about will power and so on, but those are higher order effects, it should be obvious that if there's a zero will power diet that frequently results in obesity and a zero will power diet which doesn't that's significantly more important than the idea that people can potentially starve themselves thin. The same sort of thing applies to wealth, for example, but nobody tells the poor to just acquire some will power and start spending less than they earn.

A lot of low fat foods end up being high in sugar and end up having lower satiety than "regular" foods. The perfect example being skim milk vs. whole milk. That's super important because it means that people can consume the same calories but still feel hungry with a "low fat" food. And then they are faced either with persistent feelings of hunger or to eat more until they feel full. Also, high sugar foods are easily converted into body fat, which just makes the problem that much worse.