If you accept that the 70-30 split is acceptable for sales of apps, by logical extension you must accept that the 70-30 split is acceptable for in-app purchases, because technically one could provide a free app with an in-app purchase to make it a full app, and anything less than 30% on Apple's take would gravitate the market toward this model.
If you accept that 70-30 is fair for in-app purchases, then you must by logical extension accept that recurring in-app purchases that can be cancelled at any time would also have this split, because technically one could provide all in-app purchases at a steep discount simply as "subscriptions" that can be cancelled after the first payment.
As far as I can tell, this is Apple's reasoning and rather inescapable at that.
However, I think Apple could say that a subscription purchase was 85-15 or 90-10 and they would have gotten this in with little push back.
Or even required that subscription apps cost $9.99 for the initial purchase ($9.99 is a number from my head with no basis in reality) and a following 90-10 split, or requiring that apps with a free initial release go to a 70-30 split on subscriptions. I really think they could have done that and not had a major revolt on their hands. They may not have made the same in the short term, but I think they would have had a better response and a better long-term hold on small developer and publisher loyalty to their platform.
I would personally buy a Palm Pre 3 if it had Instapaper and Kindle and the iPhone 5 didn't. I would venture that their are numerous other consumers who have similar preferences.
[Edited for clarity]