Readability has costs, and provides a service for free. They give you the choice of using their service for free (no revenue for them, no revenue for ad-driven content sites), or paying them a fee for the use of their service (which is strip everything except the content so you don't have to - a value-added service if I ever saw one). This fee gets forwarded to the content author, while they keep a commission for the service they're providing you. You, presumably, see a value in their service, or you wouldn't use it. You see a value in the content, or you wouldn't read it. They give you the choice of supporting the content and the service, where money is split to support the service you like and the content you like, or just use it without paying anything. The content author is denied ad revenue because you use a service which strips ads. This service now offers you the chance to support your favorite content authors directly, to compensate for the loss of ad revenue.
And you see extortion where?
>My entire point would be moot if Apple required 30% of all revenue >from Readability regardless of whether the subscription originated >from within the iOS app or from Readability's website
Your point is moot. Apple takes 30% from in-app subscriptions. People using the app don't have a choice between subscribing through the app or through the website, seeing as Apple does not allow linking to the website. Obviously, people using the app won't think of going to the browser to subscribe. Hence, even though it seems there's an alternative, toll-free way to subscribe, in practice there isn't. Even though it looks like it's optional, it's not, and content-based apps are going to be especially hit with this toll. The Apple 30% toll - encouraging people to develop for your platform, then putting up flat 30% fees on revenue - now that is extortion.