I do not mind a bit of political discussion around the campfire. I think it is a part of being an active citizen, provided it is done well and thoughtfully. It sounds like the fellow in your scenario went overboard, but I don't know that I would have chastised him or anything. At most, I would have tried to change the topic or guide the discussion in a less political direction.
That is, so long as he wasn't haranguing a scout. No scout's parent sends their boys with us for political indoctrination. Have to draw the line there.
(And we do not permit scouts to loiter at the adults' campfire for precisely this kind of reason. In our troop, either the scouts would have been asked to leave and go back to their campsites or the adults would have left the scouts' campsite.)
I think, though, it is a "teachable moment", as they say. I think I would look up the scouts shortly after that and just have a short discussion. "It wasn't any fun to be lectured like that, was it?" "Do you really think that's how to win somebody to your position?" "Be sure to consider the other side of the argument and don't just take somebody's word for it." That kind of thing. You don't want the scouts to think that's the way to go through life. There's a reason you don't talk politics, religion, or money in polite company.