It's safe to say that an entity/group/company in control of a market will likely act to prevent circumvention of the market.

Amazon does this - any Amazon Marketplace seller who sells products independently of the Marketplace must offer the goods on Amazon for the same price or less -- "General Pricing Rule" - http://www.amazon.com/gp/help/customer/display.html?nodeId=1...

(I believe Kindle purchases aren't subscriptions, so let's leave that aside for now.)

(I'm also leaving aside whether this behavior is ethical, good, whathaveyou.)

The problem is Apple's insistence on a 30% commission. I think the backlash against this will be substantial.