There is only one app store people can use with the iPhone though. The 30% fee seemed justifiable up until the point they released a product at a price that was literally their competitor's price minus the app store fee. Contrast this to supermarkets, where they purchase product from vendors and then mark it up for a profit. It is not anti-competitive for supermarkets to have generics, because a) there are other supermarkets to buy food from, and b) they are purchasing the product from vendors in the first place and thus taking on risk.
The other supermarkets in your analogy would be other platforms, like Android, not other app stores. A manufacturer can refuse to sell their products to a supermarket because the supermarket demands too low of a wholesale price. And a supermarket can say we’re only going to sell your products if we can’t get this amount of commission, take it or leave it. In your supermarket analogy what Spotify is saying is ‘we went to sell our products in your super market, but we don’t want to pay you anything for the privilege. We think we should be allowed to force your supermarket to allow us to set up our own booth inside your super market and then give you nothing. And if we can’t set up our own booth, we should at least be allowed to post signs in your supermarket saying don’t buy our products here for a markup, by them directly from us for a discount. ‘
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