"You know you have to crawl before you can walk".
The heights these horses are jumping didn't happen over night, it takes years of training to get their agility, fitness and balance.
Are you asking these questions in concern, or are you asking so you can actually perform???????????
These horses have been trained over many years, by professional people....who have been doing it for years,
The best mare we had was only 14.2h, now that mare had wings, she could jump well over 5', and did it often without strain or injury, had a 16.3h that struggled to clear 3'. and had a 16h that cleared 5'6" without hesitation and was eager to do so.
It is not the height of the horse, it is thier "courage" and "willingness" to perform.
To explain that horses without human intervention, (apart from the fences), can leap on their own.
I have a 18 month old filly, at the age of 3-4 weeks, she cleared a 4' fence, "SHOCK HORROR", she is fine, (and may one day be a jumper)
You ask whether it is fair to jump these heights, I think the horses in the pictures tell the story, they seem eager to me.