The kernel allows closed source software to run on it. DMABUF, being an API that can be used externally, should be exported to be used by anyone, regardless of license. What's next? GPLing the mmap() interface and making it illegal to run non-GPL software under Linux? You seriously think that's a good thing?
This has nothing to do with licenses. This has to do with AMD and Intel trying to stay ahead of NVidia by abusing their position within the kernel developer community.
AMD closed source binary drivers are no more allowed to use that than nvidia. If nvidia wants to play this game, they need to open source it.