[thetribe]

I imagine one reason why they don't want players to dunk during warmups would be a number of players who really can't dunk or dunk well attempting it just to show off. If they see a few other players doing it then they feel the need to do it too, to show that they can, but that just turns into a big mess with everyone on both teams attempting it. An attempt and no completion would be a quick way to get injured.