If you buy a Disney DVD from Walmart, there will be advertisements inside the DVD case for direct services (heck, last time I checked there were ads on the outside of the case). If you buy a Roomba from Walmart, there will be advertisements for direct parts and addons from the manufacturer. If you buy a hecking Apple Ipad from Walmart, Apple will include advertisements for its direct services once you start using the product.
People bring up this comparison all the time and it's very simply not true. You can advertise direct services inside physical products you sell at stores. What Apple is saying is not that you can't advertise prices in the store page, Apple is saying that you can't advertise alternative platforms in the app itself.
There is no physical equivalent to this for storefronts like Walmart. Home Depot does not have a restriction on whether a physical product you buy from them can have an advertisement for direct manufacturer services inside the box or software that comes with it.
If we want to be consistent about this, Apple really should be paying Walmart a fee for any app-store purchases made on devices that were bought from Walmart. After all, the user got the device from Walmart, right? Shouldn't they get their cut of app store purchases? That's how Apple sees the world.