I sell physical goods on Amazon. They charge me 15% for this privilege. The only reason they don't charge more is because physical goods have lower margins than digital goods (my margins after it's all said and done are somewhere between 10-15%). And, you know what? I'm happy to pay Amazon their 15% because they're bringing me customers that I wouldn't have otherwise.
Perhaps thinking about it like this is instructive: does Amazon have an obligation to let me sell on their platform simply because lots of consumers choose to buy stuff from Amazon? Does Amazon have an obligation to change their fee structure so that I can more easily compete with Amazon Basics branded products?
Should Apple charge a smaller percentage to subscription services that can't afford to give up 30%? Probably. It would net Apple a percentage of something, rather than the 30% of $0 that they're getting now. And it would be a better experience for Apple's customers. But it is in no way unreasonable for Apple to expect to get paid for originating sales of software and subscriptions. And it is certainly not unreasonable for them to get paid significantly more than credit card processing fees.