The issue isn't whether or not the code is open source / free software, it's specifically compatibility with the GNU GPL v2 under which the Linux kernel is released.
The GPL is picky about what licenses are compatible with it (or more specifically, what licensing terms), and for very good reasons given its intended purpose (to promote the availability of yet more GPLd code).
The GPL is picky about what licenses are compatible with it (or more specifically, what licensing terms), and for very good reasons given its intended purpose (to promote the availability of yet more GPLd code).