Just to add to that....
In the long, long deep stops debate that ended with the banning of Ross Hemingway, I was given the task of staying out of the debate itself so I could counsel Ross in the hope of getting him to stop his incessant violations of the ToS. Since Ross was very nearly the only participant on his side of the debate, and since the debate was so informative, we had to keep him in the thread, and that was hard to do when he was constantly writing things that would get anyone else kicked out. I was quite up front with Ross, telling him I was on a specific mission for that purpose. I kept him reasonably in check, but always on the border, until he exploded with all his pent up frustration and had to go.
My point in telling this is that Ross frequently accused the staff of reading our PM exchanges, and he eventually insisted we go to private email. He assumed that because he thought some of the things we was saying to me privately were appearing in posts by other staff members. That was absolutely untrue. Some of those posts were the result of obvious interpretations of things in the thread, but others came from times that I quite clearly told Ross that I would carry his comments to the staff for their consideration. There was no reading of our conversation by staff in any other way.
BTW, you do not know the potential extent of abuse by private message until you disagree with Ross Hemingway in private communications.