Also it's not a tax, though arguably it can be termed an "economic rent" (a technical term) that can be considered excessive, but I'm not sure about that.
And regulating Apple is quite different from regulating someperson. If you made a Linux phone in your basement, nobody would tell you what kind of charger you should use. But companies that claim ownership of a substantial economy and can dictate the rights and culture and economic output for a large section of society do need to have more checks on their power than just, "Well if I'm so wrong, then why do I have so much money? Maybe you should make your own phone that won't work well with anyone else's and see if you can sell it."
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.
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.
Exactly this. There's no competition, because Apple blocks iPhone owners from installing apps through any method other than their App Store (in the US, where this lawsuit is being filed). It's a programmatically-enforced monopoly.
Frankly I'd much rather change that situation than quibble about exactly how much Apple is allowed to charge for their services. Let them charge whatever they want in a free market where they have to compete on a level playing field with everyone else. If developers don't like it, they can use a competing app store to sell their software to iPhone users.