There is nothing wrong with a certain product brand being available exclusively at one store. Products and brands do that all the time.The App Store is the only store for iOS apps. The Play Store is not an alternative to the App Store because it doesn't have iOS apps, it has Android apps. Apple is making specific efforts to prevent other companies from building or selling iOS apps using their own store.
And saying the Play Store isn't an alternative to the App Store is ridiculous. They are both app stores and the vast majority of apps are available in both stores.