Maxperson
Morkus from Orkus
The DM is the Master of Rules - so, no, the rules do not require anything of him or her. A PC wants to do something, you decide if the outcome is certain or uncertain, and in the case of the latter invoke a rule and roll dice to resolve the uncertainty. That you decide the outcome is certain does not mean you're changing the rules of the game.
The DM is no different than a King. The King was subject to the laws of his kingdom, even though he could make and change law.
According to your interpretation, it's impossible for the DM to make any sort of house rule........ever. Everything he does is above the rules and therefore within the rules. That's nonsense. The fact that house rules exist proves that you're interpretation is mistaken.
They do that damage if I am not sure of the outcome, sure. But I am sure of it, at least in this example, so I don't need those rules and dice after the successful attack roll versus the 1-hp wizard - defeated PCs are knocked unconscious and robbed.
They do that damage always, unless another RULE says otherwise. Specific beats general. The DM cannot decide that damage is uncertain without an existing rule changing it or a house rule.