If your pony is obese, then follow what human trainer tell their ultra marathoners (swim, bike, and run in one day) on how to start their program. Start slowly and easy and build up on that.
Leg soundness is certainly part of every check, but not so for backs. If the rider chooses to take the tack off, then the vet can run their hand over the back to make sure nothing is happening up there. But tack only comes off for everyone when it is manditory. The vets will announce at the ride meeting which checks will be “tack off” – generally on a 50 it’s only the final check. Never saw a vet check the withers … just the small of the back, loins, and croup.
If there is a problem and the horse is getting sore, the vet will alert the rider to the issue and suggest some changes in the padding or rider position, but… I’ve never personally seen a horse pulled for it.
Naaa…that’s the really dumb thing! I think my vet is really okay with it. You know how I know?? Because he rides a pony(he got for the grandkids) once in awhile that is only about 12-13hh and he’s 6ft and 200lb. I know!! It’s sad and gross too! 
I see your point of view too. 
Ahhh. Yes. (Taking looooong sip of eggnog spiked with a generous finger of Courvoisier.) The picture is definitely starting to fade into a nice, hazy background.
(Taking another long sip) Definitely. Fading.
Hey! Will you look at that? I didn’t know I’d put twinkle lights on my Christmas tree…
