Unless they're shoving the keys into the hands of the drunk and ordering the drunk to drive, no, I do not see they have real responsibility. Can the argument be made? Sure! But, I can't hold them legally responsible, because they are not the ones committing the crime nor are they ordering the crime to be committed. I've never been to a bar that did not have a taxi on-call for taking drunks home. They're JOB is to sell as much alcohol as the person paying will buy. I can't fault them for doing their job, legally.
I also would not frequent a bar that had a bartender that let me drive home drunk, knowing I was my driver. But, it still is MY responsibility. I was sober enough to make the decision to drink another drink when I drank the drink that put me over the line to not being sober enough to make a good decision.
Well, now, that's completely different.
Here's what I think about that, as it relates to drugs. The seller is doing something illegal, and should be prosecuted when caught selling. That said, selling heroin to someone is very different from using heroin. Just like selling a gun is very different from using a gun. If a person is a convicted felon, disallowed from owning a weapon, then the seller of the gun who KNOWS the person is a convicted felon is also doing something illegal, and thus is responsible for the action of selling the gun. It is very different from selling a gun to using it in a crime. The person who uses it in a crime is responsible for the use in the crime - NOT the seller, the user. The seller is responsible for selling the gun illegally.
The drug seller is responsible for selling the drugs. I don't really care much about that crime, and think it should be drastically reduced in priority for drug enforcement personnel. I would rather see the drug seller telling law enforcement who uses it than the other way around. I would rather make the user so afraid of the consequences of use that the drug seller has no one to sell to.