Why allow someone to knowingly decrease their ability to drive?
If that were the case nobody would be driving.
- Got a bad grade on your final exam? You might be angry and drive less safely. Criminal!
- Found out your girlfriend is cheating on you and you drive away? You are not emotionally stable enough to drive. Off to jail you go!
- "Oh man! The new Whatevergame Expansion is finally out! I'll go grab it now omgomg!" You are too excited and not in your normal mental state and should not drive.
- No radios in cars and no talking to passengers. Listening to the radio or talking to people means a part of you is not paying attention to the road. You could be driving in a safer manner but you are not. You just lost your license to drive a car.
- No advertizements on the side of the highways. You are either looking at the road or at the advertizements. This is not safe. Hand over your license and your car will be impounded.
- You're been driving for three hours / need to go pee / are hungry or thirsty / bored == you are not driving as safely as you should be.
- Etc, etc
As a society we acknowledge that it is not reasonably possible for individuals to drive in the safest manner all the time. There are two options:
(1) Nobody should drive, or
(2) We acknowledge the fact that humans are imperfect and we put laws to mitigate the damage done by factors which decrease our ability to drive. This is why you can listen to radio/music but not talk/text on your cellphone, you can drive after a long and tiresome day's work to go back home, you can drink a glass of wine or a beer (but not a dozen) at a friend's house and drive back home, etc. I think most people believe this to be the optimal solution.
We have to draw the line somewhere. And I think that both a BAC of 0.00 or a BAC of 1.00+ is not where that line should be reasonably drawn.