Nobody is mixing code of different licenses. The patch is editing GPL code and publishes the result under the GPL.
What you are probably referring to is linking modules with proprietary licenses to the kernel. This is a downstream problem.
It is a debate with a long history if this is illegal. Some kernel developers believe it is illegal. Other says it is not illegal. This has never been discussed in front of a court.
The european commission has published a statement coming to the conclusion that linking proprietary modules to the kernel (static or dynamic) is not illegal:
Quote from that link:
At the contrary and in most cases, it seems that in European law the fact of linking two programs and the technology used for it does not by itself produce a derivative work: viral licensing is just a ghost. It does not exist.
The kernel developers want to prevent this linking and therefor put all the technical restrictions in place. But the GPL gives you the freedom to take these restrictions out of the source code and to distribute this “fork” under the GPL.
By the way, I am an FSFE member since several years. Yesterday I have sent them a request for comment about this specific topic. Lets see what they contribute to this discussion.