Cheering for your child to do well is one thing. This is positive reinforcement. However, when parents are just yelling at the child for what they did wrong or chastising other players, this is inappropriate. Parents who yell and scream at the referees are probably the most effective example of inappropriate behavior. There is a difference between cheering and being obnoxious.
I think that during a game the child should be left to perform on their own accord and not be criticized for their performance during the game, I think it only ends up disturbing the child and probably ends up negatively effecting their play if their parents are yelling at them.
It is not wrong for parents of child athletes to yell and scream during games. There is nothing wrong with parents yelling and screaming during a game that their child is playing in as long as they are screaming and yelling positive thing that support their children or things that they are pleased with. It would be wrong to yell critical things during a game or to heckle the children on an opposite team.
Telling a parent to be quiet because they are cheering their kids on too loud is wrong. Unless a parents is screaming at another child, then it is wrong. As long as they are yelling at their own child, there is nothing that anyone can so as they can say what they want to their child.