Tokoneki, it's an insurance thing, a liability thing. Because we live in such a sue happy friggin country, I have to post notices on my barn and my fencing that borders every road that says "Horses are unpredictable animals, approach at your own risk". Barns and arenas have to have dress requirements to keep someone from suing the barn owner or the horse owner for exactly what happened to the girl in the picture. Both of my pastures are bordered by a road that is popular among kids on bikes and townsfolk out for their evening walk. Sometimes they stop along the fence and pet my horses. I can't stop it, my horses are friendly and people are drawn to them. If some idiot crossed under the fence onto my property and got hurt by one of my horses, they could sue me (and knowing our court system, probably win) and take every penny I've got, plus every one I will earn in the next 40 years.
THAT's why people post warnings, THAT's why people have clothing requirements, to cover their own ass.
Something like that happened to me, but it wasn't as bad.
I needed stitches though.
I was riding my Shetland bareback, and I had no shoes on. Then I fell off. Got stepped on the outside of my foot, and needed a couple stitches! I was a dumb 7 year old though. I still sometimes ride barefoot, but way less often.