It is amazing how most of the world's poor subsist on rice and beans, or some other high carbohydrate staple, and yet American is pretty much the only country where a large number of the poor are obese. Yes, if you gorge yourself on rice and beans you will gain weight, but if you eat a reasonable amount it is no worse than any other food, other than being low in vitamins. We have the best fed poor in the world, and nutritious food is easy to come by.
Trader Joes has Bananas on sale 10 cents a piece every other week or so, you can a month's worth of oatmeal for about $4. Our supermarkets are full of low cost food that is more nutritious than McD's.
I have no problem with a burger now and then, but they should not be a staple in anybody's diet. Evil_Conservative is right, it is laziness that has them getting Chicken nuggets and fries for dinner instead of rice and chicken, or beans if money is really tight, cooked at home.