When he threw the elbow to Richardson's face only his right arm was being held. So why swing with your left and hit Richardson when the guy who's holding you is on your right?
And by the way he cocked his left arm his movements weren't restricted at all. I don't know how anybody can think it wasn't intentional.
EDIT: And notice how only one Heat player was holding KG (one arm then wrapped him up when he swung his elbow), yet all the Celtics fans were saying how 7-8 players were surrounding him and that's why he had to fight to get out of there.