Its because in real conversations, you talk to 1-2 other people and not a million. If a lot of people are discussing something (as they do in parliaments) people have a way of upvoting (by banging tables) or downvoting (booing) others.
In real life, I would never throw a tomato at someone I overheard saying something I disagree with to someone else -- but that is HN. By design, I assume, but still.