but that impairment never reaches a level equivalent to that of a 0.8 bac.
I think you misplaced a zero there, you're dead at 0.4.
Personally, I'm of the opinion that all driving while intoxicated laws are inappropriate. Either you're driving recklessly or not. If you are, go to jail. If not, go home and sleep it off.
The point is to prevent people from risking other people's lives by driving impared. Most people who start driving aren't likely to stop driving if they realize they are impared. This is one instance where you can't afford an ex post facto situation. That usually results in someone else getting injured and it isn't fair to subject other people to that risk. Personal liberty I think they call it.
I'm all for marijuana being legal and regulated though. Like alcohol and cigarettes, plenty of people use it in moderation and are no detriment to anyone.