The problem with your analogy is that if I see that the product I want is too expensive in Tesco (because perhaps Tesco has an own-brand that it wants to sell instead), I can go and buy it in Asda, Lidl, Morrisons, etc. That way, the businesses compete on offering the best product to the consumer, and the one that does that best thrives.
Businesses indeed have the right to make a profit on their investments. But monopolies are subject to a few conditions on that, particularly about abusing their control of one market (e.g., iOS app distribution) to undermine competitors in another (e.g., Music streaming services).
Apple is the monopoly distributor of applications and services on iOS. Spotify is right to highlight the arbitrary distinction Apple has made between apps that take your credit-card to pay for services you book with the app (Uber) and apps that take your credit-cart to pay for services you consume via the app (Spotify). Also, the rules seem to differ for video: streaming services like Netflix don't pay Apple a cut on subscriptions, so why are music-streaming services singled out?
Biting the hand that feeds IT
