(I even do this, and I prefer healthy foods when possible! But at your typical gas station, junk:healthy::10:1, which is not quite the same problem)
I believe junk food is like smoking. Everyone knows it's not a healthy choise, but they do it anyways. People are choosing to eat unhealthy food because they want to, it tastes good.
The further down the income ladder you go the more those factors come into play rather than the simplistic "it tastes good" conclusion.
Oh I believe it. I would just phrase it as: people take the easy way out when it comes to eating. They prefer something that is front of them, cheap and they don't need to cook.
You can't ignore personal responsibility. Plenty of poor folks eat health meals.
Nobody 'wants' that. What they want is good tasting food that is also nutritious. That costs more.
It's "cheaper and harmful" food.
There are a few interesting documentaries, most notably "A place at the table".