I think a lot of people are forgetting/overlooking the fact that people under the influence are somewhat at a loss for decision-making skills. Their intoxication impairs their judgment, hence they may not realize they shouldn't drive.
How many times have you had a clearly drunk person insist they're not drunk? I rest my case. The point is, if they get drunk and their judgment is impaired, the people who provided the beer have been given the responsibility to keep that
dangerous person off the road. Do you have any idea how much drunk driving stats would go up if bars/etc had no responsibility to look after their alcohol-consuming patrons?
The law started BECAUSE the last line of defense against drinking and driving is the last person who sees them before they get in the car.
Would any of you saying "the bar shouldn't be responsible" and "one adult shouldn't be held responsible for the actions of another adult" allow a drunk friend to get into their car? Your concern for other human beings shouldn't be limited to your friend- it should extend out to people in obvious, immediate peril.
Of course, many people don't think drinking and driving is all that bad. Those people are stupid assholes.