That would have been discrimination considering similar situations like this in tennis. Even Henman was disqualified at Wimbledon (of all places!) for unintentionally hitting a girl with a ball.There is no need for such a rule. As you can see in every match, all professional players, including Djokovic, know perfectly well how to appropriately return the balls to the ballkids. They also all know that they risk penalties if they hit balls somewhere without watching. Maybe it would have been better for Djokovic to get a warning at his first misconduct (as he should have). This could possibly have reminded him of appropriate court behaviour.
Oh yes, I see, I interpreted that sentence in a different way.