Most of Apple's decisions are about the end user, often to the detriment of the developer. Mandatory app review, Apple deals entirely with customers, unified in-app purchase interface.
By the way, when that policy was put in place Apple had no "monopoly" to "enforce". So it simply makes no sense for you to say that.
It had everything to do with maintaining control over their platform. Cross-compilation was not encouraged because it almost always means that the frameworks which cross compile maintain feature parity with older versions of the platform SDK. It means that end users don't see apps taking advantage of new features and frameworks.
Look at where Apple is now and tell me that was a bad decision — developers scramble to update their apps to incorporate the newest OS features, this leads to huge competition in design and polish amongst the best apps on the App Store. Ultimately the decision benefited customers and developers.
The policy was overturned largely because games became unexpectedly popular. And games don't need to use the latest OS features to deliver a quality experience. The rising popularity of Unity 3D also drove the decision. (You will recall that when the policy was publicised, there were hundreds of Unity developed games on the App Store. None were pulled and the policy was instead changed.)