To address your concern-- if two people are fighting and one thinks "I won't hit below the belt" that person is at a tactical disadvantage. Even worse if they t

To address your concern-- if two people are fighting and one thinks "I won't hit below the belt" that person is at a tactical disadvantage. Even worse if they think the other side has also agreed to that rule. So in that sense you are absolutely correct. But I invite you to think bigger. If one side lays siege to another side's city, and offers terms of surrender, the city needs to believe that the terms will be honored otherwise they don't surrender. Which is a large part of European history during the period from the middle ages up until Napoleon figured out how to use artillery, i.e. hundreds of years of examples where "fighting honorably" was the winning strategy.