Kindle: you can load books from other sources, you can also read Kindle books on other devices

Steam: you can redeem keys purchased from other stores, install apps from other stores, and they don’t mandate using their system for in app purchases

eBay: do they have any policies like this? I haven’t heard much about them or made an eBat purchase in a decade.

Amazon Marketplace: There actually is a strong argument here that they capture so much of the market that opting out is not an option. This is very similar and already under much scrutiny.

Game Console: Also very similar to this. I expect they will lobby hard on Apples side for that reason.