It makes zero sense.
According to Apple they need this revenue cut because of the cost of keeping the platform running.
If I offer a subscription in my app for 1$/month vs 10$/month what marginal expense is being incurred that apple deserves an extra $2.70 for the higher price?
On the other hand the argument is that all the $0 apps are being subsidized by the paid apps, so they went percentage based so that the cheap apps didn't have a huge fee. It would mean the cheapest paid apps in the app store would be a lot more to absorb the fixed cost, or if they shared it evenly, then there would be no free apps.
If there were competing app stores available on iOS, then we could say they are really entitled to that profit. But we don't know how much of this profit reflects the value they brought versus the rent they were able to get due to gatekeeping the only bridge into iOS-land. So no, we can't for sure say they are entitled to the entirety of that profit.
McDonalds, Nike and Gillette do this too. Hey, maybe you’re doing it yourself: is your salary really the cheapest way for you to produce the work you get paid for?
How do you allow free apps but still collect money? Maybe you could make a distinction between free and paid apps, and charge a flat fee to the paid apps, but then people would just make their apps free and process their payments separately anyway; essentially the same problem you hit with the 30% fee.