Steve's sixth and most important reason in that essay is "We know from painful experience that letting a third party layer of software come between the platform

Steve's sixth and most important reason in that essay is "We know from painful experience that letting a third party layer of software come between the platform and the developer ultimately results in sub-standard apps and hinders the enhancement and progress of the platform." That's Steve saying he wants to 'protect' the app store, and justify the 30% fee. If people could pump out Flash games it would have put Apple users off buying apps...