The "poor fat people are just weak-willed" argument is SO despicable. Americans did not suddenly get poor habits in 1976 http://en.wikipedia.org/wiki/Obesity_in_the_United_States The obesity problem was caused by a change in the environment (and, yes, the increasing power of the food industry is one of those causes) and only a change in the environment will fix it.
As a fat person I, to an extent, disagree with you.
Yes you're right about environmental changes, and that new changes could solve, or help solve, the problem. That doesn't mean that my fatness isn't related to me being weak-willed. I absolutely have the ability to lose weight and get fit, but I'm too lazy to do this. I know people who love food just as much as me, but they chose either not to eat everything that they want to, or to do extra exercise to make up for it, or both.
Maybe there are people out there for whom it isn't about weak will. I know genetics play at least some part, and obviously environment as well (if you grow up with parents who only feed you fast food for example). But I'm confident that for a lot of people, weight issues are about being weak-willed.
I'm not saying that we have become more weak willed - i.e. that this is the reason people are fatter than 100 years ago - just that weak will does play a very big part.
Maybe if I was born a century ago I would be skinny and fit, but that doesn't change the fact that my current weight is caused by my own lack of will as well. Will power is the reason on an individual level, but not the reason behind the cultural trend.