*Sigh*... All right, I understand your point of view. They're not your acquaintence, they're not your concern, and I certainly see how approaching a severely drunken man in a bar is a shitty idea in the line of... arguments. But you can still report the license plate to the police. It's not stepping in unjustly if someone's life is at stake, which it always is when people drive drunk.
It's not your decision if they decide to get behind the wheel. It is your decision if you decide to not call the police. This isn't about sticking your neck out for people who are or are not your friends. It's great that you don't associate with people who would do this. But a drunk driver is a threat to anybody he drives near, and himself. You can say it's not your fault if he takes his own life into his hands. And that's fine. But he has so many other peoples' lives in his hands too.
Say somebody you love is driving cautiously, abiding by all traffic laws. Suddenly, blindsided by a drunk driver and killed. Police go to the bar where the driver just left and talk to people there. One person there says "Yes, he was definitely drunk. I asked if he was driving. He said he wasn't. I don't know him from Adam, it wasn't my responsibility to do anything else." Is that going to satisfy you, honestly? Will you be able to sleep well knowing that whoever it is you loved is gone forever because somebody drove drunk and people who were in the position to get involved and perhaps prevent it refused to be proactive? The drunk person's decision to drive is not yours. The decision to get involved and prevent them from possibly destroying lives because of that irresponsible decision is yours.
Quoted for emphasis, spot on. This reminds me of the bystander effect. I will simply never understand how anyone could walk away and decide he/she has no responsibility in such a situation. I would never sleep at night if I knew I could have made an effort to do something but chose not to. I think that's the point here: in the example of someone about to drive drunk, it requires minimal effort to pick up the phone and call the police, at the very least. There is simply no good reason not to do so.
HM: Very visually noisy, your face. It's like a brass band under a wig, marching towards me every day.