LOL, I used to stand on my horses when I was a kid while trying to pretend to be a trick rider. I kept trying to get them to walk and they'd look at me like "freaking idiot, sit down and THEN I'll walk."
I don't understand it as a selling point though. I'd rather see a video of the horse loading calmly into a two horse straight load!
People do it because it works. It's sad but if you go to a sale and the bidding slows down the person riding it will stand on the horses back and get two or three more bids. I used to sell a few horses every year at catalog horse sales and I taught my oldest son how to ride them in the ring and the last thing he would do is stand up on thier backs and we would get 2-300 dollars more because some fool thought it made the horse more broke. The fact is that the horse can't see you very well so they mostly don't know what you are doing anyway.
I will occasionally do it for fun but I don't understand it as a selling point at all.... we have a few apple trees & scout will eat apples off & I will stand on his back & pick them high up the tree hah