Sorry, but I respectfully disagree. You've oversimplifying the complex subject of nutrition.
It is now known that it's not as simple as "calories in - calories burned = weight gain or loss". The body burns more calories processing some kinds of foods compared to others. The body is less efficient or less encouraged to convert some kinds of excess calories into fat, depending on the source of the calories. It is becoming more clear that insulin plays an important role in a large percentage of cases of obesity, and it turns out most of the most cost effective foods encourage the body to produce the most insulin (and, almost paradoxically, also encourage hunger).
Your own example demonstrates my previous point. I would need to eat 18 (!) of those salads you suggested, every day, to merely maintain my weight. Or 5 Big Macs. It's obviously an extreme example, but which do you think poor people would choose? Buying cheap food can make the difference between paying the rent and living on the street.
With all due respect, it's ignorant and offensive to blame poor people for all their problems. Having an adequate amount of money makes an astonishing difference in your ability to achieve good nutrition, lower your stress, pursue advanced education, get enough exercise, etc.
I know it's popular to blame all poor obese people for their own predicament, but it's simply ignorant.
I agree with your first paragraph - there are significant corrections to the rule of "calories in - calories out = weight gain". That doesn't mean it's not a good first order approximation (in my experience, within about 15% [1]). It also doesn't mean that fat people aren't eating too much. If you are 40lbs overweight, eat less. If you are not overweight but don't have a 6 pack, that's the time to start worrying about exactly what you are eating.
Also, you are correct - the straw man you are arguing against is wrong. It would be cost inefficient to eat 18 salads and 0 big macs.
Now, lets say you are obese. That means you are eating 6 big macs. If you are constrained by cost, you could reduce consumption to 5 big macs (saving the cost of 1 big mac), and you will eventually be merely overweight rather than obese. Or you could change your consumption from 6 big macs to 4 big macs + 1 big salad, and you will approach a healthy weight (and save money). If you want a visible 6 pack, you might even need to play around with the exact composition of your 2250 calories.
You are also ignoring the fact that it isn't very expensive to cook healthy food at home. Last night I cooked about 6 meals from rice, lentils and frozen vegetables. Total cost: about $5. 6 big macs costs about $18.
Also, as for exercise, the lowest earners have the most free time and they exercise the least (I won't even get into the vast majority of poor who don't work at all). They spend the most time watching television. The highest earners have the least free leisure time and spend the most time on exercise (and the least on TV). I'm not sure why having more leisure time would make it harder to get exercise or cook a healthy meal.
[1] My personal experience: I was obese as a teenager. I got myself down to a reasonably healthy weight simply by reducing intake during a 1 year period when I was living well below the poverty line (this was during college and due to some unusual choices I made). I was hungry as all fuck. My rate of weight loss was within 15% of what calorie counting predicted.
It is now known that it's not as simple as "calories in - calories burned = weight gain or loss". The body burns more calories processing some kinds of foods compared to others. The body is less efficient or less encouraged to convert some kinds of excess calories into fat, depending on the source of the calories. It is becoming more clear that insulin plays an important role in a large percentage of cases of obesity, and it turns out most of the most cost effective foods encourage the body to produce the most insulin (and, almost paradoxically, also encourage hunger).
Your own example demonstrates my previous point. I would need to eat 18 (!) of those salads you suggested, every day, to merely maintain my weight. Or 5 Big Macs. It's obviously an extreme example, but which do you think poor people would choose? Buying cheap food can make the difference between paying the rent and living on the street.
With all due respect, it's ignorant and offensive to blame poor people for all their problems. Having an adequate amount of money makes an astonishing difference in your ability to achieve good nutrition, lower your stress, pursue advanced education, get enough exercise, etc.
I know it's popular to blame all poor obese people for their own predicament, but it's simply ignorant.