To be fair - anyone contributing to the GPL ought to be aware of the terms of their license. They're not "appropriating" something that was consciously and deliberately licensed by the contributors for these exact terms of ownership.
These people probably did nothing wrong, but sanctions are sanctions and they apply unilaterally. Their best path of recourse is to fork Linux (which is entirely legal) and continue their work on a downstream basis with patches that can be merged into the main branch.