This is a really interesting thing to consider. I find it interesting you say the centre of the shop items are too expensive. It's a real issue for the poor especially as a lot of the crap in the central aisles is a fairly cheap source of calories: bread, instant noodles, frozen meat products such as mega-bags of sausages as well as the cheaper brands of biscuits. Fresh fruit and veg as well as meat, particularly the healthier lean meats, are costly, especially to feed families.
In the US it's easy to make an argument for healthy foods leading to savings in medical costs, but when you've been raised on crap food you have a taste for it, the fresh stuff might taste weird to you, which obviously would put someone off buying it, regardless of health benefits and long-term costs. The health costs argument does reveal a kind of endemic short-termism that a lot of people seem to suffer from, or which may feel or be necessary if you only have a tiny budget to work with each weak.
A problem in the UK is that health costs don't work as an argument for 'investing' in healthier foods, because medical treatment is covered by the NHS (although there is always talk of charging people for treatment of 'lifestyle diseases'. Though there is the argument that you get to actually be alive and well for longer and have more quality time with family and friends.
Obviously I don't think that all obese people are impoverished, all though highest obesity levels do occur among the poor and less educated. For those that are doing fine financially, I think there's an aspect of addiction to sugar, plain laziness in not wanting to spend time preparing food, or a sort of mental refusal to acknowledge or to really relate to the fact that a crap diet will make you sick in the long run.
While I definitely think this behaviour belongs on the wall of shame, and I do believe people need to take responsibility for their own health, I think that there are underlying issues with government and business that need to be pinned on the wall for this one too.