Maybe because trolls and jerks have no goals beyond heating the discussion? Most calm and constructive forums and mailing lists I’ve ever seen had an unwritten rule (written actually, but who reads ‘em, right?) to ignore “hot” messages or at least the hot tone in these. It doesn’t prove that it’s the only way, but once the reply is done, every other user feels urge to add to that, because it is in a human nature that something said repeatedly or upvoted has more weight than something stated once, but it is harder if you’re first (crowd psychology). It is a culture of a public place (a thing that supports healthy cooperation) and they have to learn it, no matter how strong is their desire to respond.
One forum I'm on has a very strong community standard of "This place is like a local pub. If you show up mouthing off you'll be called on it by the regulars who may all look like they're shit talking each other, but who have mostly spent time together in real life, and who as a group have each other's backs against outsiders. If you keep it up you'll be asked to leave, possibly if needed by the managers (forum mods) who'll ban you for a short or extended time."
It works remarkably well for that particular group of people. It's almost certainly turned a lot of people away who _may_ have pulled their head in and become contributing members of that community, but largely they don't care too much. The forum has stayed small (it was recently characterised only a bit unfairly as "12 cranky old cunts" by someone who wouldn't/couldn't live up to the community standards there.)
