Posting of what the Member believes to be any personally identifiable information (email, real name, phone, address, etc.) about another User or posting the contents of a private message is a privacy violation. For example, if a Member posts that "XYZDownDaDrain is actually Bob Smith", whether the information is accurate is irrelevant. The Member was trying to disclose personal information. This is a serious violation and can get you suspended or terminated from the site. Posting or referring to the contents of private messages without the author's prior consent is considered a violation of privacy and can get the User suspended or terminated from the site.
But, if one was to post the contents of a PM, without saying that the contents posted are from a PM, who is in charge of finding out that the post contained the contents of a PM?
And if the sender of the PM is in no way identified, where is the violation of privacy?
When responding to a message, you have two buttons. If the two buttons were labeled "Public Reply" and "Soon to be Public Reply", then I would agree with you.
Consider that, in all probability, the only way admin is ever going to know someone has posted the contents of a PM in the first place is because the author of the PM has howled to admin about it, it's probably safe to assume permission was not granted by the author.