dont be fool by manipulative title la.
they r not forcing apple to allow sideloading app, rather allow 3rd party payment option.

ie, u wan sign up subscription from the app in your iphone.
right now, payment channel hav to go through apple's own, basically.
user -> apple -> developer/company
in which apple middle man takes staggering 30% cut by providing the platform only.

however, a lot of these service already existed as web sign in, b4 they even had an ios app.
hence they already hav their own mature payment gateway which they very much like to use.