Walmart is the largest retailer in the world. Making their shoppers used Walmart Pay is how they get data about their purchasing behavior and stay competitive with Amazon (which also doesn’t accept Apple Pay). When you’re a behemoth, you can get away with not always pleasing your customers. The good news is that if you want to use Apple Pay, there are still some retailer alternatives depending on the category you shop. For pets, Chewy is amazing! And they take Apple Pay.
Amazon is different in the fact that they essentially are not a brick and mortar store, in the sense that Walmart is. They followed Dell's early model, where they didn't have a brick and mortar store (I know by extension you could say that they did with Borders/Waldenbooks in their early days), but they really don't, except in very few locations.
Also, to even counter that, by extension, they do accept Apple Pay. Keep in mind that Amazon also operates inside Whole Foods, and they definitely accept Apple Pay.
BL.