Without jumping to assumptions and conclusions, how would you go about discouraging these situations? Beyond encouraging individuals to make responsible decisions (which, curiously, you did not address here), would you want laws prohibiting the suppliers? Or, are you thinking more marketing aimed at the consumer?
I don't think there's an easy answer to the problem. I think the problem is largely one of social norms- we profess publicly that drinking and driving is a huge deal, but few people even think twice about letting someone drive after two drinks, even though that'll put a lot of people over the limit. And again, situations like office happy hours after work are a near certainty to lead to several people leaving the establishment with keys in hand while over the legal limit. Yes, there should be personal accountability, and there are always alternatives available, but people aren't going to actively seek out those alternatives because for the most part it doesn't particularly seem like what they're doing is wrong. Social norms, including all of the unconscious signals sent out by the establishments serving the alcohol, suggest that there's little wrong with having two beers with dinner and then driving home.
What's the answer? There certainly isn't an easy one. Alcohol is deeply embedded into our social fabric. The best way to eliminate the problem is to change those social norms. That's a very hard thing to do. It certainly cannot be accomplished by legislative fiat- any attempts to do so would be at best useless, and at worst would backfire and only entrench the problem even further (see also: prohibition).
The one thing I could think of would be changing the financial incentives of the service industry so that they don't make a huge percentage of their revenue from alcohol sales. As long as liquor is the highest-margin item on the menu, they're going to keep pushing it as aggressively as they are legally able to. That's the free market at work. If the margins were much smaller, we might see less drinking with dinners, which would eliminate a lot of "casual DUIs". Of course, even here we run the risk of things backfiring- if, for instance, margins are reduced by making drinks cheaper, it might instead lead to MORE drinking with dinner.
I don't have any solutions to the problem, I'm merely trying to face it honestly and objectively identify what it really is. The problem isn't that we have a bunch of bad people who are actively running around doing things they know are bad. The problem is that we have a society that has placed a high value on alcohol and creates situations where people don't even really consider what they're doing a "bad thing". After all, everyone else does it (and, by and large, they're right- as I said, I am convinced that a huge percentage of the population has, at some point in their lives, operated a motor vehicle with a BAC over 0.08). The uneven enforcement only adds to the problem. I understand that the enforcement is stochastic- that the people who drive the most miles drunk are the people who are the most likely to get caught driving drunk- but such a low percentage of drunk driving incidents are ever caught that enforcement often winds up seeming capricious and unfair. If everyone who ever drove while over 0.08 wound up getting a DUI, our DUI problem would be solved pretty much overnight. Instead, only a fraction of a percentage of incidents are caught, and each time someone isn't caught it only emboldens them further and convinces them that what they did wasn't really all that wrong.
Now, again, I'm not trying to remove personal accountability from the equation. Implicit pressure and social norms are one thing, but we are still ultimately responsible for our own actions. I can recognize the environment that emboldened someone into making bad choices without resolving that person from the consequences of the resulting bad choices. And I'm not trying to say that DUI is not a big deal. In high school, one of my best friends was orphaned by a drunk driver. She spent three months in the hospital. Her life was irrevocably changed because of some impulsive decision by some random guy. That guy suffered the consequences of his actions, and he fully deserved to, because he ultimately made a choice. Still, I can by sympathetic towards him. He probably wasn't a bad guy. He certainly didn't intend to cause harm. Despite the fact that he killed two people, I do not view him the same as a murderer like Aaron Hernandez whose actions were premeditated, where death was the desired outcome from the beginning. He might not have even thought he was doing anything wrong. Or maybe he did- I certainly cannot see into his mind. Punishing people after they have already violated the law, though, isn't the most effective way of stopping the problem. In order to stop the problem before it happens, we have to look frankly at the root causes that lead to it in the first place.