I thought at one time I had read that they should never be ridden. Just looked it up on the AMHA website and they state that riders under 70 lbs are okay. But if the average weight for minis is 150-250 lbs, a 70 lb rider seems like a lot to me.
Obviously this girl is much bigger than that if her feet are touching the ground. People need to use some common sense. I used to try to ride my german shepherd when I was a kid. It probably looks about the same.