Firstly, she is not the only one to emit these noises, they're nearly all at it - even Murray sounds as if he's having difficulty with a 'stool' when he is serving:-)
Secondly, it is not cheating - only if the player is still screaming as the opponent is about to play their shot. If she has been warned in the past, i dare say that is the real reason why.
Finally, if a player refuses to face her because of her screams, that player would forfeit the match.