Similarly, once I have bought something from Walmart I can use it as I wish. Our business transaction ends there, so your analogy isn't really apt.
> Does Walmart let you sell your product in their store and say you can look at it there but get it cheaper from Amazon?
Funny you say that, because Walmart and many other brick-and-mortal retailers will happily price-match Amazon and each other. You know why? Because they are not a monopoly or pseudo-monopoly and so need to do good by their users to compete.
Of course you can justify Apple's behavior any way because you can claim that I am on an iPhone so I am on their property or something and so they are my overlords but that is precisely what users here are trying to argue against.
Or to be honest, you don't even need to justify it that way. The magical market justifies it because the fact that these apps are on the Apple ecosystem means that staying on it is better for them than staying off it. And no other justification is necessary. And you would not be wrong.
But people have a moral intuition about these things based on how they see the world work, and so they have an intuitive sense for when something seems 'off', even if the market seems like it's working. That's why they complain against things like exorbitant pay-day loans despite them too being an example of a market that seems to be working.
Last I checked, I did not get an iPhone on lease from Apple. This attitude where just because I am on an iPhone means I owe Apple in perpetuity needs to die.