"isn't that irresponsible? I think you have a moral obligation to stop them from going down the street and attacking someone else who doesn't know Aikido"
I think you also have an obligation not to use more foce than you really have to.
You can't be held responsible for that persons actions. What if he does something the day after? Is it your fault then? If not, at what time does you obligation cease? 2 pm?
Are you moraly obligated to render him unconscious? I think not. To break his arm? Nope. Should you stop him from attacking you? Yes.
The thing is, you can't be sure he'll start a fight with someone else. You can't just knock him out because you think there's a chance he MIGHT.
Also, the police are the ones responsible for peoples safety. Not the local Aikidoka.
Just some thoughts...