Another thing to add on to what Maura has said about the kids at MIE's barn... I don't know what level these little girls are at, but I tell my beginner jumping kids to just sit still and let the horse do his job, not just b/c they're on hunters but because they're learning how to jump and they need to learn how to be still before they're solid enough to tell the horse what to do. Perhaps others will disagree. But when someone is first learning how to do a posting trot how much are they "riding"? They aren't. They're learning how to post. Then when you first learn how to canter, how much are you "riding" the canter? You aren't. When they get more solid they can do things like steer and post or balance the canter. I don't think learning to jump is all that different. Also, I don't think it's just hunters that should learn to leave their horse alone. I was eavesdropping on one of the best GP Jumpers I know school her kid and she said "get your work done in your corner and then you can just let the horse do his job".
Is this what causes people to learn to perch/pose? Maybe. Maybe they just keep jumping higher without turning into an "advanced" rider. Maybe there are a lot of coaches out there who learn what form is supposed to look like by looking at other kids at shows and matching them instead of reading Hunter Seat Equitation.