Engagement in self-moderated threads would likely drop significantly if such a rule were implemented. Lets consider instances where users with valid but differing opinions from the thread creator (OP) might hesitate to post, not only because their replies could be deleted, but also because any merit earned could be revoked. This creates a poor effect on open discussion. Every suggestion has two sides, and often, those who push for stricter rules end up being affected by them eventually.
When you visit a self-moderated thread, there is a line on top of the OP that says:
So, most of us already know the rules of a self-moderated thread, and we know that any of our posts in such a thread can get removed anytime, and it's up to us whether we want to post there or not. Those who know their views and opinions differ from the OP of a self-moderated thread, and they know they are going to write something that could get deleted, they shouldn't post it there, but should create a new thread and write their post there and give a reference to it, not making it self-moderated so that everyone can post there. We often see this happening, and there is nothing wrong with that.