I don't believe it's always lifestyle choice. I think one of the big problems is how cheap and how much calories are in fast food. You could go to the store once a week and throw down around $100 for groceries and maybe have meals for two weeks. Go to a fast food joint and you could get the same amount meals for half that. If people could afford good food I think obesity rates would go down but with the living wage around $17/hour and the minimum wage half that people have to cut corners somewhere and with the convenience of fast food that's where they will head. More than 1/3 of The urban poor by the age of the 3 are overweight or obese. So I'd say a lot it has to do with income levels than a lifestyle choice.
Regarding fast food, there are ways to reduce the calories you take in. For example, I never eat the bun with any hamburger - I'll get a double whopper instead and use those patties as the hand-holds (with a napkin, of course). That's my lunch once or twice a week and I haven't seen any negatives arise from it. Need to drink something? Get water. It's nice and free.
And why does anyone "need" french fries or onion rings on a regular basis?
Choices. It all comes to down personal choices.
I think the point of Carthonn's post was more that poor people are filling up on the cheap staples such as the hamburger bun and fries. For someone who doesn't know where the next meal will come from, they would be inclined to eat the whole supersize meal.