It's because in-app subscriptions are really just recurring in-app purchases that can be cancelled at any time.
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.