Here’s the thing. Apple is paying for the servers that process the in-app payment, the credit card processing, handling all the charge disputes and chargebacks/fraud requests from the credit card company, etc.
That’s not a $0 Bill for Apple. Credit card chargebacks cost businesses a significant amount of money, as well as employing the iTunes App Store support agents, data center infrastructure, they designed and built Xcode, add new features to iOS that enable new features in the apps, handle push notifications, etc.
First of all credit card fees don't cost anywhere near 30%, stop exaggerating. I would be surprised is Apple pays anything more than 1-2% fee not their credit card transaction. And chargebacks are billed back to the developer, Apple does not lose money on chargebacks.
And the yearly developer fee of $99 in no way covers all of that. So they have to take a cut somewhere.
For Enterprise customers (like Basecamp probably is), it is $399. Still not a lot, but not the same as an individual developer.
I have been given this some thought since the original story came out. If the concern is that companies will take advantage of Apple (as much as you can take advantage of a company with $1.5T market cap). How about Apple offer a new level of developer account:
Apps are still posted on the App Store
Initial PURCHASE of the App would be handled by Apple,.
Apps can use their own payment system in App
Subscription/In-App purchase must offer a tangible product or service (i.e. no Loot boxes, power-up, or feature unlocks that already exist in the App.)
Apps can offer a subscription system and be able to advertise/link in App.
Developer must meet certain requirement to be accepted (existing product, certain financial requirement, etc)
Apple can then charge either a much higher yearly fee ($10,000? $50,000) or could charge based on App submission ($10K for initial submission, $2k for each update). That should cover Apple's costs to process the App.
I bet many large companies (Microsoft, Amazon, Spotify, and maybe even Basecamp) would be willing to pay these fees to provide their users with directly links instead having their users have to find an alternative signup location on their own.