Because you are not forced to sell exclusively at walmart. You can sell your product at any store and people can get to it. Because of the walled-garden nature

Because you are not forced to sell exclusively at walmart. You can sell your product at any store and people can get to it. Because of the walled-garden nature of the iPhone and iPad, the only way for people to get apps and make purchases for those apps is via the App Store. So, in your comparison, you would only be able to sell at walmart.