Same reason any company can't set whatever they want. Vast majority of companies aren't deemed monopolies so this doesn't apply to them but this restriction holds over them once they grow to a certain size nonetheless. Even the most ardent capitalists/free market advocates agree that monopolies have to be regulated by the government.
The real questions are what makes company a monopoly. You can always argue you aren't a monopoly but it's what convinces people that makes most sense and many people are beginning to be convinced that Apple/Google etc are monopolies in certain markets.
I don't see an answer to the question you quoted in your post?
It sounds like you are implying that a monopoly classification would somehow be relevant to fee/pricing strategy?
The reality is that every company that has a hardware/software ecosystem has some form of AppStore. Sony, Nintendo, Valve, etc. Should we consider them all to be monopolies of their store? For fun, let's say they all are.
In order for that monopoly classification to have some impact, it would have to be used in an anti competitive way.
Let's say Nintendo wants 20%, Valve wants 30%, and Sony wants 40% of an apps sales price. You adjust your price for each platform, the software costs more on Steam Deck and PlayStation. Are Valve and Sony using their stores in an anti competitive way by charging more than Nintendo? I don't see how. It is not anti competitive for Sony to charge developers more.
An example of something anti competitive would be Sony telling you that you can only put things up in their shop if it's exclusive to Sony.
This is a misunderstanding of free market advocacy. The status quo for mobile apps is not a free market. Free markets are defined by meaningful choice. If there are undue restrictions on switching providers for any service -- it doesn't matter whether the restrictions come from the government or from private companies -- it's not a free market.
Same reason any company can't set whatever they want. Vast majority of companies aren't deemed monopolies so this doesn't apply to them but this restriction holds over them once they grow to a certain size nonetheless. Even the most ardent capitalists/free market advocates agree that monopolies have to be regulated by the government.
The real questions are what makes company a monopoly. You can always argue you aren't a monopoly but it's what convinces people that makes most sense and many people are beginning to be convinced that Apple/Google etc are monopolies in certain markets.