My understanding is that that would be legal. The problem for those companies is they can't build and distribute a Linux kernel that contains those patches, bec

My understanding is that that would be legal. The problem for those companies is they can't build and distribute a Linux kernel that contains those patches, because then that is a derivative work. So in practice they have to release their changes under the GPL as it's not feasible to ask users to compile their own kernel with their custom patches.