TOURNAMENT TROUBLES!

You are a bit confused...
In the first case, you really do have to kill the entire group of neutrals, even if by repeatedly attacking the last ones. That is, systematically killing all groups.
In the second case, only the unupgraded neutral units are killed, we flee, and the entire neutral group disappears from sight. Thus, in this case, the upgraded stacks are obtained "for free," and that is not good.
And the tactics end up being completely different, not identical, as you claim.

I am not claiming at all that by weakening I will take out even one stack, in either the first or the second case. It is enough for me to blast a couple of times with Chain Lightning or Meteor Shower or Armageddon. And flee or die. In the first case, the neutral's army will be weakened only by the number of those killed; in the second case, the upgraded ones will all disappear — have I understood this point correctly or not? If yes, then so what? They are upgraded — they are smarter :) Therefore, they understand that the artillery preparation was not accidental — so they need to get the hell out, or else it will be worse. In this case, the player resolves the situation for themselves — either use artillery preparation, but lose leveling points for the main hero and at least 2500 gold, or not use artillery preparation, but get full leveling from the battle and not lose 2500 gold.
In principle, it seems illogical to me that carrying a neutral without penalty is allowed (and this is clearly a bug). Moreover, by using it, the player in the game suffers no material losses, unlike using the neutral elimination trick (here they do suffer material losses for it, see above). Nevertheless, this trick is banned. If we look at the big picture, then either both of these tricks should be allowed, or both of these bugs should be banned :)