There's multiple differences here - one is that progressive taxation is based on the idea that if you're rich, you're using the commons and the services society provides to a greater extent, thus you should contribute more. On the other hand, there's nothing such that Wal-mart or Kroger is providing for you, they're simply taking a bet that you'll not walk away in disgust at seeing the price they toss at you.
Furthermore, you're overly optimistic in that financial dispositions are the only factor entering this - you could pull off tricks like noticing a customer really seems to like one brand of say, mayo and start cranking up the price, or even more atrociously, identify things which look like a necessity and amp up their price... or ,for instance, amp up the prices for people with full baskets because they're less likely to leave if they've already picked up a ton of wares.
The whole practice smells sky-high of bullshit, and I'm certainly not going to visit any store that has electronic price tags.