That's generally because any disagreement, argument or dissenting opinion is characterised as flaming, and the thread gets locked. The community's nice because of the people and because the mods maintain order, but that order does come at a price.
Unfortunately some posters can't seem to manage a dissenting opinion or disagree without calling the one they disagree with retarded, or an idiot, or stupid. I just deleted three posts and locked a thread due to less than appropriate nomenclature. I love the protesting private messages I've gotten in the past: "But they really are a retarded idiot. You can't warn me for telling the truth!!!!!!"
When we see a thread is heading in that direction, should we let it continue and have people complaining because they got a warning and posting ability suspension, or shut it down before it goes that far and remind people doing stuff like that isn't a good idea?
I remember a Community Discussion thread about "How would you react to your child telling you they are gay?" that was, even with the autocensor in place, a great discussion, complete with disagreements and dissenting opinions and only a few trolls who were ignored and reported. [At one time, due to the way less than mature members were using the word gay, it was in the autocensor.] Because it was a great discussion and everyone was participating instead of ranting and bashing no one got a warning for bypassing the autocensor. One of the few times that rule went by the wayside.
You can disagree with something, or someone's opinion. Doing so in a respectful manner is what can be hard. When someone has a vested interest in a topic it is easy to take an "attack" on the topic as a personal attack, and snap back in turn, and it snowballs from there.