As criticism of this condition has mounted, we now have Steve Jobs responding to an e-mail from Tao Effect's Greg Slepak on the topic, sparking a discussion between the two on the change.
Jobs pointed out John Gruber's recent analysis of the change, calling it "insightful and not negative" as compared to the knee-jerk reaction in the first few hours after the SDK agreement surfaced. The revised viewpoint suggests that the real reasons behind the move are to maintain innovation and quality as more and more apps are written for Apple's touch platforms; meanwhile, we've also heard a somewhat plausible technical explanation-slash-rationalization for the move.
After Slepak read the piece, he responded in turn: "I still think it undermines Apple. You didn't need this clause to get to where you are now with the iPhone's market share, adding it just makes people lose respect for you and run for the hills.... From a developer's point of view, you're limiting creativity itself. Gruber is wrong, there are plenty of [applications] written using cross-platform frameworks that are amazing, that he himself has praised. Mozilla's Firefox just being one of them."
This from Jobs, and the echoing statement that's in Gruber's article, both largely ignore the fact that plenty (most?) of the 85 million users buying and running applications on the Touch OS (whether on iPhone, iPod touch, or iPad) don't care how those apps are created as long as the app experience is compelling -- they wouldn't know an IDE from an SDK, or be able to tell Xcode from Flash on a bet.
As fellow TUAW staffer Mike Rose points out, in the case of Unity, "that platform is enabling game development that would simply not be taking place otherwise on the iPhone." Right now it's not clear whether Unity is on the good or the bad side of Apple's new rules, but if the philosophical argument against third-party tools holds water, there are lots of apps already on the store that may be in trouble.
Assuming that users 'wouldn't like' apps made with those third-party tools, and that Apple is therefore justified in protecting the platform from crappy apps, strikes us as more than a bit paternalistic -- especially after the onslaught of fart apps and the recent Bikinigate, it's hard to accept "Apple knows best!" with a completely straight face.