That's what I admire about John Carmack. He wants people to hack his games and do stuff with them. He pushes to release old engines as GPL (soon the Doom 3 engine will be released, offering some real game code to study for people wanting to get into game development)
Id games have lot's of artwork, levels, etc. that is not released under GPL. In minecraft it's users that create the content.
Id can release their engines under GPL without harming their sales.
