Apple is not preventing you from buying iOS apps from anyone but Apple. Spotify and other companies are preventing people from buying directly from iOS apps by not offering the option for users to purchase their product with in app purchase API. Spotify uses Apples resources to distribute their apps but then complain about it. Anyone who is distributing an app on behalf of someone else is going to charge for the service and why shouldn't they. IMO the question isn't should they charge it's how much is appropriate? And people already have a right to install whatever software they want on their device at their own risk as it should be.