The rule is pretty simple to me. Don't hit up high on a QB any time he is behind the line of scrimmage or any player in the act of trying to catch a ball.
I think you're way off. The NFL sent a "training video" that included Ray Lewis hitting a receiver in the process of catching a ball and it was the example of legal.
So it's not that easy. And MANY defensive players have been flagged for form tackling QBs and hitting them in the stomach, no where near the head.
The rules are NOT simple. Why do you think so many people are saying publicly, "I don't know what's legal and what's not anymore." And it's not just a few people.