No one needs to explain it because it's irrelevant. It's about that winning does not make war crime or crime against humanity magically stop being war crime or crime against humanity. To reduce to absurd for the sake of example, if US develops a weapon that allows it to kill the entire population of China and uses it in a war then US can't possibly lose that war but it would still be an unthinkable crime against humanity and the punishment should be appropriately severe (it's an absurd example so I can't even think of what it could be, probably US should stop existing or the entire government be put in whatever the IRL equivalent of Azkaban for lifetime).