If you make a new phone OS tomorrow and lock it down to a single store with a 99.99% cut, no one cares and it's not illegal. You don't have oligopoly.
When Apple or Google get a little too strict with their app stores, things get a lot more ambiguous.
The problem is that Apple require vendors not to advertise, link to, or even mention that other payment methods are available outside of the walled garden.
That's the stunning uppercut. The size of the fee is merely a follow-up kick to the nuts.
No doubt Apple's PR team have been feeding Bloomberg background that pulls them towards only thinking about percentages, because "high prices bad" is an much easier story for lazy hacks to write than "Apple's T&Cs are distorting a marketplace and forcing vendors to mislead consumers", and much easier for Apple to leak about and subsequently respond to without addressing the core problem, especially when they're eventually forced to sit down and haggle with consumer/anti-trust regulators.
Because people don't have a choice. If you want to build a successful software as a service offering in 2020, chances are you need to offer apps for it. Those apps have to be available for iOS and Android else they're dead on arrival, especially for iOS.
While Android lets you easily sideload apps (but even that is rarely enough as most users won't deal with that), iOS is a very strict walled garden. You have to play by the rules, and the rules are finicky and vague.
Like in the Hey mail saga, Apple's whim can make or break an entire business. That's not okay.
By obliging merchants to mislead the consumer about pricing, Apple have very likely distorted the marketplace. Which is why half the competition regulators on the planet have a file open on Apple right now.
So no, corporations do not have carte blanche to do whatever the fuck they want. See also: Uber.