Poor Fat Bastards
The reason that makes poor American the most likely to be obese has little to do with the diffusion of McDonald's and other fast-food chains. Or better, the fast-food phenomenon is only a manifestation of a bigger underlying issue.
The issue is wealth.
My own family has rather humble origins and they (grandparents and parents) suffered some hardship during WWII. They did not starve, thankfully, but their meals weren't very rich. When after the post-war reconstruction Italy had its economic boom in the 50's and 60's, suddenly they were able to buy much more food than ever before. And having abundant meals, rich meat roasts, lasagna dripping with oil, was seen as a liberation - liberation from the near-subsistence diet of my grandparents' youth, from the hardship of the war. It was almost a celebratory rite for the beginning of the Age of Wealth. Of course all that eating made my family fatter (except my dad; he's naturally skinny and very active).
Fast forward a few decades. I visited Indonesia, I've met people from China, the Philippines, Africa... and you know what they have in common? Few of them are fat. In fact, the rubbish scavengers of Jakarta tend to be on the wiry side of things, much like a lot of the immigrants from North Africa. And even in those places, when you see someone big, most times they belong to the middle or higher classes.
Food has never been so cheap and easily available in human history: this is the true root cause of widespread obesity.
For centuries we had either to spend a lot of energy hunting our prey, or gathering fruits and vegetables (Ever tried feeding yourself with the fruit picked from a tree? You should...), or growing them from the soil. We used to spend a lot of the energy introduced with food just to get enough food - and for most of human history only very wealthy people could afford to buy enough food and do very little activity.
But now things have changed. Most of the poor* in developed countries are at least well off compared to the starving Africans or North Koreans: they can afford to buy all the food they want or like.
Enter the fast food. Meals served there aren't really evil, but they present a pretty high energy density - meaning that they pack a lot of calories in a smallish amount. They also are quick (that's more or less the whole concept of fast-food, no?) and pretty cheap. It is also my impression that especiall McDonalds has meals designed not to provide long-lasting satisfaction, but this is a different story.
Moreover, supermarkets often offer low-cost (and doubtful-quality) foods - I still remember with horror the low-cost sausages I bought at Tesco's in London; thank heavens crap like that is nearly impossible to find in Italy - so that you can fill up your fridge with a pretty small amount of money.
The poor are more likely to eat junk food for a couple of reasons. They often are ignorant and uncultured: that means they know little about the characteristics of various products, why it is better to have a varied and blanced diet, why one should limit the intake of certain substances... they lack the knowledge to make informed choices.
But another factor is the spiritual poverty, which is not strictly correlated to material poverty. Is the dysfunctional families, the teenage single mothers, the absent fathers, the lack of parenting, the hopeless life on welfare... who would ever bother to buy fresh food and cook it, there? No, the solution is to take a trip to the nearest fast food or pizza joint and munch on that stuff.
Yes, there can be some cases in which corporations may have played a role by deliberately misrepresenting the nutritional qualitiees of their products, but in my opinion this contribution is small. It is also true that where diet traditionally includes more vegetables and seafood obesity appears to be less common - and people grown eating less calcium (as milk) and proteins are also smaller.
Nanny-statism, regulations and the whole lot won't work unless the state assumes exceptional coercitive powers - that cannot happen, you think?
The only long-term solution will be either deliberate modification of the human body to make it able to deal with a much richer diet and much lower level of exercise; or to wait for evolution to do its job and produce a species better adapted to its environment.
*The definition of poverty is arbitrary. Often, the poor are by definition those with income below a certain fraction of the national average.