How many times have you seen one team knocking the crap out of another the entire game... and get away with it? Turns out, there's a reason. Human nature.
A study done by Indiana University finds that teams are knocking you around early can get away with doing it later in the game.
One caveat: that study is about the NCAA, but it certainly applies to all of basketball. Human nature in officials is human nature. If you come out aggressively from the jump, the officials will let some of it go... especially if you make the other team look like it's being passive. If a guy is just standing there and you've been the aggressor the whole game... and you go over his back... you can get away with it. But if that guy has been boxing you out the whole game and you go over his back the same way.... you'll get called.
Does any of this sound like the Hawks game to anyone? The Hawks were the aggressive team and they probably went over the back a few times. But the Celtics stood there the whole game rather than box out... so Atlanta was allowed to keep crashing and making contact without repercussion.
So the message is... as Chuck likes to say when he plays dress up in his wife's old cheerleader outfit when she's not around.... be aggressive, B-E aggressive!