Take the #AppStore. Apple blocks third parties from offering rival app stores for its #iOS platform, which means you can only install apps that have been blessed by Apple. That blessing is contingent on the software authors involved giving $0.30 out of every dollar you spend in their apps to Apple.
2/
This has two effects: first, it makes certain products impossible to offer. The gross margin on an audiobook is 20%. Apple takes a 30% cut of sales. Try to sell audiobooks in an app, and you'll lose money on every sale. That's why non-Apple audiobook stores like Libro.fm and #Downpour require you to buy your books in a browser, which hamstrings them and gives Apple an unbeatable advantage (Apple doesn't charge itself 30% on every transaction, obviously).
3/