Usually that thin layer of source code is under a proprietary license as well. The main reason to have it is actually not legal but technical: distributing kern

Usually that thin layer of source code is under a proprietary license as well. The main reason to have it is actually not legal but technical: distributing kernel modules in binary form (.ko) separately from the kernel itself is a pain in the arse (bordering on impossible) since they depend on the exact version of the kernel and in some cases even the build config (e.g. skbuff size may depend on #ifdefs).