[plq]

the driver interfaces are exempt from gpl. that's how e.g. nvidia can publish closed-source drivers.

however;

1) those interfaces are in constant flux (a situation officially supported by linus himself) which makes it more difficult to maintain proprietary drivers. if you open-source your module and it becomes part of the mainline, it's migrated to the new interface by the dev who introduces that change.

2) once you load a proprietary driver, the kernel is considered to be 'tainted'. this means people are much less willing to listen to you when you report problems. (try dmesg | grep -i taint if you're using a proprietary driver)