Rules are only made with considerations of situations of conflict, not for situations of harmony, and your rule regarding private messages is completely obvious in situations of harmony, and becomes impossible to fairly enforce in situations of conflict. It's exactly the type of rule that can be exploited for terrible abuse.
Imagine a conflict situation regarding this proposed rule. What would enforcement would look like? The only case where you could get conflict are situations which are impossible to moderate, because you would need to examine the private messages of the two users. These are private issues of trust between two users, which we cannot legislate. This is an issue of personal ethics, not an issue of site policy. I agree with the personal ethics, by the way, which is why I try to make it clear that I am a blabbermouth to people before they speak to me privately.
An hypothetical example of such a situation; User A posts a paper, and sends a private message to expert User B saying "I know page 37 has a mathematical error in it, but please bear with me, it will get fixed soon in a later version". No new version 4 months later, user B writes a review pointing out the mistake, which user B would have discovered independently. User A complains about violation of privacy, user B gets kicked out. It's this kind of nonsense you have to consider when rulemaking, not the usual day-to-day business of people working in harmony. That's why the only real stringent tests of rules are situations of real discord.
While it seemed like the worst thing that could possibly happen at the time, our terrible discord might have been a blessing in disguise for the rulemaking process, as we now know how the rules work in situations of conflict, and they aren't so bad. On balance, they produced fairness to everybody (more or less), and cooperation of all (with enough time). With your rules, they would have gotten me blocked, and VK banned.