Wouldn't simply not providing old versions of the driver achieve the same thing then?

And beyond that, what's the logic? "We don't want black hats to diff our patches so we make sure that the legit users can't get patched firmwares in the first place"? Doesn't really add up to me. People with enough knowledge, time and resources to pull such an attack will manage to get the binaries one way or the other.

I still think that incompetence and laziness are more likely causes.