And if Hey is typical of the vast majority of SAAS services, they pay affiliate fees and other marketing fees based on a percentage of sales to third parties that promote their service. So why not play Apple their 15% for a seamless in-app purchase system to add new users with?
Hey has responded very specifically to this https://hey.com/apple/iap/
Their main objection to this is that they want to own the relationship with the customer. They want to be able to issue a refund or account credit or discount or hardship exceptions, which you cannot do through Apple's App Store.
If they had their app distributed elsewhere and they were losing customers, they could choose to work with apple, but right now they have no choice. At the same time, apple would have to earn its relationship with developers like target and Walmart have to do with their suppliers.
There are plenty of companies and individuals that have chosen to direct-sell rather than offer products through other retailers. It's a choice they can make, foolish or not.