Are they just getting him used to light tack?? I assume no-one is getting on! The more different experiences youngsters get the better: rugs, plastic bags, being rubbed with a whip, halters, ropes - why not a very light saddle and girth? It looks 'wrong' as it seems the precursor to mounting - but assuming that is NOT remotely on anyone's agenda then can't see how it is harming the foal?
Mark Rashid's 'Old Man' used to fashion a foal harness when they were newborn and carry them around in it - it apparently made their acceptance of tack much much less stressful later on.
With my shetland foal I used to stand astride him (no weight on his back) while giving him scratches and rubs to get him used to the idea of people being above him.
Where did you find the pic? Do they explain what they are doing and why?