In comparison to Apple, phone user : Shopify merchant :: app developer : this SaaS business. If I as a consumer think that Apple policies are too stringent, I can pay money to buy an Android phone. But the cost to me as a merchant of switching from Shopify to a custom site where I can choose who connects over API, once my business is underway and has built a brand identity, is absolutely prohibitive.
App approval processes are app approval processes, plain and simple. And if you make revenue share a precondition of using your APIs, you don't have open APIs.