Its the same point of having exceptions and EXPORT_SYMBOL / EXPORT_SYMBOL_GPL you don't necessarily WANT everything to be affected by the viral nature of the GPL. Sometimes you WANT things to be able to freely interface with the kernel without necessarily being open source.
Originally Posted by GreatEmerald
Think of the X server.
KMS is a GPL interface, The X Server is MIT licensed. We have closed source drivers though because they replace the GPL parts with their own code and then interface through the X server.