They only open-sourced them once their value to Id was approximately zero. It's still a really cool thing to do, but neither Id (or any other game company I'm

They only open-sourced them once their value to Id was approximately zero. It's still a really cool thing to do, but neither Id (or any other game company I'm aware of, anywhere, ever), has released their game as GPL while they were still making significant amounts of money from it (which in practice means within the first couple of months, for almost all games).