I couldn't find anything immediately helpful on the web so I downloaded
the source code for grub. Fortunately, there's only one instance of the
phrase "incompatible license" in there and it's in a small function
called "grub_dl_check_license". The purpose of this function is to
confirm that a module being loaded has a license of "GPLv3", "GPLv3+" or
"GPLv2+". If not, then it returns the above error. I'm not intimately
familiar with the GPL, but as GRUB is one of GNU's higher-visibility
projects, I imagine this is seen as a perfectly cromulent thing to do.