This would be a great guideline — developers should offer IAP to buy content, since it’s so easy for users that they’re likely to make more money overall with it. But forcing all app publishers with purchase systems outside of IAP to suddenly and completely adopt it in parallel has no apparent practical or pragmatic justification. Instead, it just looks like greed.
If in-app purchase is so great, publishers would adopt it voluntarily, and the market would determine how much extra customers are willing to pay for the convenience. He also expands on the point I made earlier today:
One issue is that this policy assumes that all apps are made by someone with the ability and authority to collect IAP payments on the service’s behalf, which isn’t the case for third-party clients using a service’s API.
If Twitter charged a subscription fee, or even sold any content whatsoever, no third-party Twitter clients would be permitted on the App Store, effectively preventing that entire market.
Arment, correctly I think, says it’s a mistake to focus on whether the policy is legal or 30% is “fair.” The consequence might well be to prevent the creation (or continued existence) of many potentially great apps, and that’s bad for customers, developers, and Apple.