It's only arbitrary in the sense that, yes, it's Apple's business what it tries to sell in its store, like every other company, but it's not arbitrary in the sense that the App store wouldn't have existed if Apple tried to do what you suggest would be "fair." You see, for example, Uber/Lyft, etc., already pay the driver around 85% of the fare, and on the remaining 15% Uber is still losing billions, so if Apple decided to, which they could, charge Uber 30% of each fare, it wouldn't exactly work out mathematically if you know what I mean. So, what would happen? iPhone users wouldn't have the Uber app available to them, which would be crushing for Apple as well as Apple is reliant on iPhone users being able to use the most popular apps, e.g., Facebook, Uber, etc. Indeed, having the most apps in the beginning is what enabled the iPhone to take off. Remember Apple's early ads "There's an app for that."
That's why these decisions need to be made by business people, not people who "feel" a certain way. You are focused on some sense that competition needs to be fair, when nothing is further from the truth. Indeed, all businesses in a free market are based on trying to get an edge on their competitors. Is it "fair" that Amazon decided to get into the accessories business and can sell its cables on Amazon.com without having to pay the same costs that Anker and others have to pay. Yep. That's the way it's done. And yep, it's up to Apple and everyone else that builds a "store," what they want to sell in it, and what they want to charge people to pay to sell in it. A friend of mine works for Kellogg's. Costco is now one of the largest seller of products like cereal in the world. Costco dictates to Kellogs that if it wants to sell Cheerios to the pre-screened and lucrative tens of millions of customers that Costco has spent billions to acquire, Kelloggs is going to sell to Costco at a very low profit margin, is going to package a certain size box, and is going to stack them on pallets a certain way, etc. Oh, and by the way, Costco is still going to undercut Cheerios and sell its generic house Kirkland brand of Cheerios at a lower costs because they own the ecosystem.