I think a large chunk of Instapaper's revenue comes from the iOS apps so assuming Apple continues to keep acting like a douche, Marco would probably just as soon cancel the subscription plan rather than let his app get rejected.
That's the most obvious route for Marco - the (rhetorical!) question was directed at Apple, because they appear to thwart a large domain of what iOS could be used for.
Instapaper is exactly the kind of app this is directed at. It is a SaaS subscription. It would be foolish to turn away all the additional sales you'd get by being able to offer subscriptions within the app (something you couldn't do previously, forcing Marco to take paypal.)
Sure, I'd be curious to see if it's economical to hand over 30% of whatever subscriptions he receives to Apple.