So much of the value they paint of the iOS ecosystem is in the apps - 'There's an app for that' was a large campaign last year. Yet they're now threatening to make a number of their more popular apps economically unviable according to their creators.
In-app payment will mean a new app, no? Which won't get approved if it contains a rant against Apple on that screen explaining the price hike they need to make it viable. Yet the current versions contain links to web-based payment, which would be relatively easy to update to include a page saying 'Apple are thieves' and explain why.
I'm well aware that many / most customers would see such a message, give a stereotyically teenage whatever reaction and move to a competitor, but a good percentage won't and we've seen how fast bad news travels now. Enough providers that customers value enough are now angry enough with Apple to try their luck at this, particularly with a competing platform available and growing without the same restriction, and I wouldn't like to bet on Apple winning this.