LOL. "No rhyme or reason." The App store was created by Apple to bring the wealthiest, most educated, screened, customers in the world, together in a secure, privacy focused environment of screened apps, to provide access to digital goods.
Apple carefully planned it and spent billions of dollars to build it and now spends billions of dollars to maintain it. That is Apple's business model. You may want to have the authority to order Apple to get involved in the sale of cars, furniture, landscaping services, attorney services, etc., but we haven't sunk that far yet.
You are also so far from the truth when you say Apple gets to restrict how Spotify can collect payments. That's simply not true. Spotify can, and does, collect payments directly from consumers. What Apple does, and every company in the world does, is set the rules if you want them to carry your product in their stores, e.g., most companies are going to charge you 50% of your sale price to carry your product for them. Ebay, Etsy, Amazon, Google, Microsoft, etc., all have fees, rules, etc., that you have to follow if you want to use their platforms.
Some companies, for example, decide it isn't worth it to put their products on the MacOS Store, that's their choice. Some companies, like Amazon, put on their big boy pants and don't allow in app purchases because they don't want to pay Apple a commission.