Technically, they state you must have the same or better offer. Better isn't the same as cheaper, it can mean more expensive with more features, so an "premium iOS offer with added features" for 30% more could be justified :P
But I still think the idea is to avoid what would be perceived as an Apple tax when buying subscriptions in-app.
They might back down from this like they did for the dev license changes, but this habit of always changing the rules in the middle of the game regardless of who gets hit is really getting old.