Interesting thoughts... the more I think about it, the more I think the captain in this case made a fairly serious error. Engineer put it very well.
A captain, rightly and necessarily, has the final authority on their aircraft. It's important that this is not seen to be abused, and imposing a schoolteacher-type collective punishment is an abuse, IMHO. Leave the business of catching & punishing the culprit to the authorities... whatever your opinion of their efficiency, they are professionals at their job just as you are at yours.
FWIW, if I had been on that flight (as a fare-paying pax, not deadheading) I would have been strongly tempted to open the door myself, if safe to do so, and bring charges of assault and false imprisonment against anyone who tried to stop me.
I understand his frustration at the smoking situation, but did the captain in this case, I wonder, pause to consider that his actions in detaining the entire planeload could easily have resulted in an air rage scenario which would be much more serious than the original smoking issue? I'd like to hear some input from cabin crew on how they would have felt about having to enforce such orders from the captain...