Because Walmart owns the physical space but apple doesn’t own the physical device.... Apple has set up shop on the real-estate (the device) that it sold over to the user. I’ve seen this argument plenty of times and it rings hollow to me every time.
(That may not even be 100% hypothetical. They have lots of money, so can afford getting delayed payment for phones. They might even be able to become a global phone company, too, and rent you a phone and a network subscription)
Plus, you could unlock your phone (in some cases) and move to another carrier, which theoretically meant a different store! True freedom.
Of course actually using these phones and putting apps on them was MISERABLE, mostly because they were symbian and/or j2me apps