That's nonsense. They don't charge for Quicktime, iTunes, or Safari. Someone made a decision that they needed to start charging for XCode, probably so they could make some extra revenue off of iOS developers.

What makes it that much worse is that XCode is built on top of a lot of open source software, like gcc and dtrace.

The bigger problem, though, is that if there is a non-free barrier to the development environment, the software becomes that much less free. There's not much point in giving the software away for free, if everyone you give it to has to spend money in order to build it.