I'm really surprised they don't get sore backs from all of that seatbone pressure--maybe they're using therapeutic/advanced pads that spread the pressure out over a greater area. Or flex-panel saddles. I've watched riders here "sit on their back pockets" but those are generally in a Western saddle with a huge tree, mitigating the effects.
It may be functional but I find it exceedingly unattractive. Hard for me to believe that the rider isn't often "behind the motion" of the horse, causing him to work harder. I saw the style used by some of the riders in Spain during the Al-Andalus ride and had trouble photographing it.
In 1980 Bob and I rode in the South African National Endurance Ride. In my book I noted that almost without exception the riders sat back in the saddle at the gallop. I have numerous pictures of them showing the same riding style as the winner of the women's race in the UAE. Julie