A horse's conformation is judged with the cannons plumb. If someone showed me a parked-out photo I would ask for a proper conformation photo, just like I would if the photo was of a horse grazing, or dozing with a cocked hind leg. It is some kind of Saddle Seat convention that probably has some historic reason, possibly related to driving.
What it would mask is what the horse's true topline is, and its true hind end conformation is. Among other things probably.