I consider private in meaning that that what I receive is private inasmuch as I can do what the heck I want with it. IMO, if I send you an e-mail/PM/letter, it becomes your private property, and no one has a right to read it, unless YOU decide to share it. Once I have sent it, I have basically given up all ownership of it. If I trust you, I may possibly reveal stuff I don't want to become public, but if I don't know/trust you, I have only myself to blame if you share it with others.
Otherwise it should be called confidential mail - which would be completely different.
It may be a rule (although I have yet to see it written anywhere), and it is certainly not accepted, if you look at the vote.
No need to issue threats - I have brought up a subject for discussion, and immediately you start issuing veiled threats - Are you sure you are not Dr. Pol Pot in disguise?