The legal permissiveness described is critical for most software because it enables business adoption. But for a kernel, which sits at the intersection of hardware and software, "free as in beer" is not enough, there is another critical factor: how easy it is to make it work with proprietary hardware. A free kernel that won't work with my custom chip and whose maintainers won't accept the necessary patches to fix this state of things, is just useless to my business.
The legal permissiveness described is critical for most software because it enables business adoption. But for a kernel, which sits at the intersection of hardware and software, "free as in beer" is not enough, there is another critical factor: how easy it is to make it work with proprietary hardware. A free kernel that won't work with my custom chip and whose maintainers won't accept the necessary patches to fix this state of things, is just useless to my business.