I read/heard/imagined(?) that the reason GPU vendors weren't willing to hand over their source was due to a proportion of their performance being software based. Which then a competitor could also implement.
Closed source drivers make it more difficult for patent owners to check, and launching a lawsuit without evidence of infringement is very expensive.
But they don't need to provide that source, just hardware specs. Gallium3d already has shader compilers and schedulers.