Because Musk didn't draw attention to himself the way Kalanick did during the NYC cab driver strike. Also Kalanick tends toward the image of someone trying to do away with labor unions whereas Musk is seen as someone trying to save the planet.
Arguably, because Uber is in the spotlight as a result of breaking the taxi worker strike at JFK. Don't think Musk is getting a pass per se, as much as he hasn't attracted attention to himself.