Can't we just leak the spec?
Anyone can then implement opensource driver based on that and distribute it freely, since NDA won't apply to them.
No, for the resulting open drivers to not be legally dubious the spec can only be obtained by doing a clean-room reverse engineering.
Just because something is accessible publicly doesn't mean it's suddenly legal to copy it, same as it isn't OK to go into someone's house just because the door was open. Unless you're police for some weird reasons.