Parents are hoping their children will play in college and even in the pros. There is even more TV coverage of little league sports these days. I've heard about soccer leagues that have instituted a policy of 'no yelling' for parents. It seens commendable but maybe not realistic. Sport Illustrated, July 24th edition, devoted an article to this issue.
Parents are too invested in the games.
I think having guidelines for parents is important. No yelling may be too drastic. There are a lot of horror stories about overzealous coaches. In my experience, that is exaggerated. I think most coaches have the best intentions.