After all, we all lived long enough without their software, we can live without it a bit longer.
Yes it's a gift, but it's also code that has to be maintained and updated along with the kernel, so it's not really 100% a gift. If you then consider that they might keep on selling a proprietary version of the code (which - don't get me wrong - is 100% legit and fair) they might also get basically free labour: they could rebase onto the latest public gpl version, they might get notes of various issues and bugs...
Quite literally, it's free labour.