I was recently at a show and the announcers had to continuously ask people to move back from the rails. They were leaning over into the ring, waving their hands about and flashing cameras and stuff really close. Some horses were getting spooked. The thinking could be that a non-horse person may not know proper etiquette at shows. They may not realize what things can upset a horse and/or rider during an event. There is no reason why shows aren't for non-horse people. I think they can be enjoyed by everyone.
The show host is responsible for teaching ettiquette. They should have printed information handed at the gate and posters or signage hanging on. Extra volunteers to help people who can't read or comprehend what they read.
And I'll emphasize, not just the "rules" but the why. Too often we say "don't do this" without bothering to explain the reason to help people understand why it is important.
Personally, horse shows, especially dressage, are the LEAST spectactor sport friendly events that two non-horsey parents would get in the car and drive to go see.
"Hey Walt, let's wake the Beaver and Wally up early tomorrow and drive to the fairgrounds so we can pay parking and an entry fee to watch a local dressage show."
"June, I think that is a great idea. what's dressage?"
"Oh Walt, it's where those sophicates wear their evening clothes while riding horses."
"That's right. They ride in circles and do straight lines. I get it. Dead quiet. You can hear a pin drop."
"That's what I am talking about."
"I think it sounds like a wonderful way for a family that has never been around a horse to spend their Saturday. I'm sure the boys would rather do this than a ball game or motorcross event."
"I do too. It will give the boys some class and help them rise above their station when they go back to school on Monday."
"June, I love how you think. We should call Eddie's parents. If anybody could use class it would be that boy."
"Oh WALT. You are soooo funny."
I personally would wager that most poor behavior are from the families of the horse people exhibitiing, because no matter how many times you tell Grandma, she can't pass up a Kodak moment to fire off an instacube.