I've recently heard this term and from what I can tell, it describes what has become a pet peeve of mine. The pet peeve being posters trying to tell people what they should or shouldn't post. Every so often on a message board, I'll run into a poster who feels the need the lecture others on how they're posting. That the situation doesn't merit their reaction or that their choice of words makes them hard to take seriously. I've have this happen to me one time when I was just letting off steam and had someone come to tell me that I wasn't disagreeing "respectfully" even though I never attacked anyone.
But here's the irony; the thing that really pisses me off about tone police? Their patronizing tone. These people talk to others like they're teachers or parents talking to unruly kids. There is something about people who feel like the unsolicited referee of a discussion that gets under my skin. It's like they think they have the moral authority to tell others what to do.
So in a way, I don't have a problem with pointing out someone's tone. If someone is being an A hole, they should be called out on it, but this holier than thou bullshit gets under my skin. (if that makes sense).