As pointed out in the OP comment, it's basically 'money for jam' by the point he releases the source code:
> It's an entirely different thing; he made a thing, sold it, and then when he couldn't sell more of it, gave it away. That's nice!
Carmack has extracted as much profit as he could care for from the source code. The releasing of the code is warm fuzzy feelings for zero cost, while keeping it closed source renders zero benefit to him.