With a product like Hey, you're signing up for their email service. You might use that service on your desktop, web browser, Android phone or iPhone. It's more than a little nuts on Apple's part to try to force a product like that through the iPhone system, especially because if I start using an email account I'm going to continue using it whether I have an iPhone or not.
Should Microsoft get a cut if I connect to my email account through a desktop app?
The mobile app is merely 1 of many interfaces to the service. In Jason's case, he's arguing that they are setup to run a business to offer service to their customers...all of their customers. Apple forcing him to fracture his users into iPhone vs everybody else is just bad business before you factor in any cut at all.
While I partially sympathize with the ostensible desire to be able to offer great customer service under the guise of refunds for unexpected downtime in SaaS apps, it's hard to see how this won't be abused by developers: Won't every app just offer App Store refunds to convert customers to 3rd party payment?
I suspect that's the actual reason these things are wanted. I completely understand that many people think 30% is too high (and maybe it is!), but it's hard to see how 0% isn't too low.
What would be his reaction if Apple enabled all these customer service options he supposedly wants, but at a cost to the developer? For example, a code for first-responders to download the app for free, but the developer still pays 20% of the store price. Or a customer refund, where Apple keeps 20% instead of 30%. After all, it's not about the money, but actually about offering great customer service. So he'd go for that, right?