LA's James appeared to strike Pistons centre Stewart in the face as they battled for a rebound, leaving him with a large cut near his right eye.
Stewart then repeatedly had to be held back by team-mates and coaching staff as he tried to retaliate.
Both were then ejected from the game after an official review.
"Everyone in the league knows LeBron's not a dirty guy," said James' Lakers team-mate Anthony Davis. "In fact, when he knew he hit him, as soon as he did it, he looked back at him like, 'Oh, my bad. I didn't try to do it'."