The situation you are describing is not a penalty. It is after making contact with the QB the defender launches himself onto the already going down QB that is a penalty. Again it has nothing to do with blockers or hits on the QB, it is the blatant attempt to injure the QB that draws the flag. It is also a penalty to dive onto an already down ball carrier I suppose you think this shouldnt be a penalty either. It is the same thing as launching yourself onto an already going down QB.
I would like to see a replay of a defender just landing on a QB that was flagged. Every play I have seen flagged for this you could see the defender make an extra effort to launch himself onto the QB. Just falling with/on the QB is not being penalized.