Because we think everyone is innately good, their actions don't define them.
It's analogous to someone saying something is stupid without the person actually being stupid.
...At least that's the way I see it
I had an INFJ friend tell me once that "something was wrong with me" because I didn't believe in capital punishment. To further persuade me, she gave the classic "what if the murderer killed your family?" example, and I told her that although I would be completely distraught and lost without my family I'd find the strength to forgive that person and believe in my heart that the crime in itself was punishment enough. She still didn't really get it.
We strive for peace and will do anything to acheive it. Even if that means protesting, rebelling, being argumentative, etc. to get our point across.But that said it's mostly a matter of semantics; I think that we are indeed prone to smooth things out, but not in a let's all agree and wub each other Fe kind of way,more as in always provide the balance when the balance is off. Sometimes this requires stirring things up. We love conflict sometimes, because it can restore harmony.
I like the way you describe our role as the one who establishes and maintains balance. That's better than our portraying the thin-skinned peacemaker stereotype.