If we're arguing about imaginary teenage hackers to fit whatever argument we need...nobody, they're running a hackintosh on a $200 netbook and they're pirating XCode anyways.
I imagine this will be like xcode for iphone only working on intel macs. You could develop (but not release) on PowerPC mac by changing a couple plist keys, and apple still sold a lot more macs anyway.
Remember there are places in the world where you can get a Mac but can't get an Apple store account.
Maybe nobody. Maybe it belongs to Mom or Dad.
When talking about teenagers (the 15 to 19 year old crowd), they may not be the ones buying the Mac directly.
I also think this is an unnecessary and dumb idea.