Walmart does have competition across town, unlike apple store. I can got to Target, Costco, Safeway, et c. Unless they collude to price fix, they will indeed have to compete by driving down their margin.
Moving "across town" from the apple store involves an IT project and hundreds of dollars in order to save dozens of dollars.
Moving from Walmart to target requires lots of supply chain logistics. I suspect it’s actually easier to move apps to new App Stores than work out how to get products into completely different vendors.
Also for IT, Walmart requires IT interfaces from its suppliers.
Moving "across town" from the apple store involves an IT project and hundreds of dollars in order to save dozens of dollars.