Personally for me I don't see it as gift, he licensed out the engine but didn't want to be in the engine business, after selling enough it feels he just put it out there so it's his stamp forever with the GPL infection. I think he already felt the diminishing returns at the time. He knew about the sharing of floppy discs and hacker scene and eventually someone would've done it and I think he felt cornered and said fuck it might as well put it out there to beat them to it.