Why is there so little team-kill in this game?

This game is really popular, and offers free choice to have friendly fire, How is it possible that so few people enjoy doing this given how many strange people there are around? How do you control something like this? It would only take 20 people to ruin the fun for hundreds of people every week. Is there any psychological effect? I've only encountered a team killer once in Hundreds of hours, The best part is that they can't even be banned.