"Arbitrary and Capricious"? Pretty much in line with what other platforms charge, so I guess the entire market is "arbitrary". Capitalism does dictate that you can charge whatever you want, if you are willing to lose money in the process.
"5X to 10X too high"? That would be comparing the Apple Store to what credit card processors charge. Apple provides WAY MORE value added than just processing credit card payments: authentication of purchases so people don't steal software, a search engine, curated apps so most/some of the junk and malware is screened out, tech support to developers and consumers, advertising for developers, automatic app updates, subscription services, coupon codes, TestFlight beta testing, etc. At 15% I'm guessing they lose money.