They're exploiting their near-monopoly to expand their ownership and control of user actions and make more money. They know most of their users won't notice, and many won't care.
It's absolutely bonkers how much Apple gets away with today, when Microsoft got torn apart for abusing their monopoly 20 years ago for things that weren't even 1% as abusive as what Apple is doing here. Microsoft got attacked for bundling an internet browser with their OS. Can you imagine the outrage if Windows started swapping Google URLs for Bing ones or iTunes links for Microsoft-owned properties?