More than likely you can buy the same brand at another store such As Target, etc. If Walmart was the only outlet where one could buy the product and that product was also competing with Walmart products, that is an unfair advantage in Walmart's favor.

In the case with Apple Music vs Spotify, Spotify is not only at a disadvantage of API availability, but also how the rules are not applied for Apple's own apps (marketing push notifications come to mind). On top of that, it is more difficult to complete on price as you are coughing up 30% for any IAP (which you have to use Apple's IAP APIs to begin with). So spotify either charges an extra 30% to cover the fees or eats the 30% to remain price competitive.