We have that law currently and it doesn't stop drunk driving. Until people become rational actors (which they manifestly are not), we cannot just stop at prohibiting outcomes, but also must prohibit means to some degree.
So people will ignore laws against drunk driving, but will obey laws against drinking in the first place. Interesting theory, if only we had some historical evidence to see if that would be likely to work...
He's saying they don't anticipate the damages they will cause driving drunk, so there are laws against drunk driving (the 'means')
I don't think anyone is arguing mj should be outlawed because of driving, or that alcohol should be either, the point is there are careful considerations that must be made and you can't just remove all regulations and expect things to go swimmingly
