Dimitrij ovtcharov's tomahawk serve toss

In Dima's matches, I see that he uses the tomahawk serve a lot and he is tossing the ball very sidewards and it looks pretty obvious that it isn't a legal serve. But the umpires don't fault him for that toss. Why is no one stopping Dima from his sidewards toss? 

Hi Jasper,

This is a good question.  The key is the height that he is throwing the ball.  There are tolerances as to the angle of the throw and since he is throwing it so high the umpires still deems it to be almost vertical.  When he throws is sideways the distance comes more stark to the eye.