Let's say the subscription cut was 10%. Everyone would release their apps as free, and charge for everything as a "subscription" because it's less than the 30% cut for app sales or non-auto renewable in-app purchases.
Basically, there is no other percentage that Apple can use here, for very good reasons. If they lower the percentage for this, they have to lower all the other percentages.
And that's not going to happen, because 30% is pretty reasonable for what they provide. The Android Market takes 30%, and Amazon takes 70% for Kindle subscriptions.
Edit: IMHO, everyone talking about the percentage is missing the point. The problem is the policy itself. It's just too general and broadly applied for a huge amount of stuff out there. Marco nails the issues here: http://www.marco.org/3437484678
It's not much different than if Google were to decide that all AdWords clicks are now $3.
The thing is that nobody cared when it was a one time fee for their $1.99 app. When Apple starts talking about subscriptions that may cost 10's or 100's of times that each month, Apple starts to look like their overreaching - and it could be damaging to long term adoption of the platform.
One might even hypothesize that these ham fisted policies are defining the very nature of the offerings in the app store - it explains the preference for fart apps over sophisticated (expensive) products.
Apple is just coming off as if they didn't think this through.