Every WalMart I've been in makes you walk past the produce section (which isn't overly expensive... it's WalMart) to get to the processed junk food aisle. Just sayin'!
I do think the environmental influence means a lot. If your parents, your neighbors, role models, people on TV, whatever, all look and eat a certain way, then you end up not knowing any better. It's one of those weird human behavior things... you may very well live your whole life in a land of opportunity but if you've only ever lived in a neighborhood, where people simply don't take advantage* of those opportunities, then with no clear example for you to imitate it's actually pretty hard for an individual to break out of the mold.
That's not sympathy, charity, pandering, enabling, blaming, or anything else. It's just one of those traits of human behavior.
Back to what "food" a lot of people buy at the grocery store, this is kind of a chicken and an egg thing. As more and more people have chosen to buy shitty, cheap, food that makes them fat, our food industry has adapted to provide more and more shitty, cheap, fattening food. Healthy food hasn't gone anywhere though and it's not as expensive as people seem to think- especially when you don't blow half of your food budget on diet coke by the case, gatorade, alcohol, "contains some fruit juices," and other (wait for it) shit.
* can't/won't/don't/know better/don't know better is beside the point