Posting IC messages for public view, especially messages that are private, or of limited distribution, can easily cause problems. Yes, they can be shared elsewhere. But that doesn't mean that sharing them there, or here, is the right thing to do. And perhaps if we don't allow it here, that may make people think twice about sharing them anywhere.
In addition, I think I agree with jaune regarding even realm-wide messages. Just because a message has been posted to a realm doesn't mean that it is in general circulation.
