If you look at it in a such black-and-white perspective of the terms, then yes, it doesn't make a lot of sense. The thought behind the suggestion, though, is why should Pokemon B rake in the reward if Pokemon A did all the work?
A more exaggerated version of the example would be that Pokemon A literally brings the opponent down to 1 HP (assumed with intent to KO) in so many rounds, doing all the damage itself, then the following round, Pokemon B steals the KO with Quick Attack or something. Something about that just doesn't seem fair to me. Should Pokemon B get credit for the KO solely for the reason that it packs a priority move or is faster than A? I don't really think so.