There are two opposing schools of thought, that I have observed when it comes to training riders and horses.
The idea that you’re training the horse to perform, almost regardless of the rider.
vs.
The idea that you’re training the rider to perform, so that they can pilot almost any horse.
Most people want to show (and win) before they can get to the latter stage, as it takes awhile to get good enough to be able to not make horrific mistakes. So - trainers train horses in ways that they’ll make up for their riders mistakes.
When I was a young aspiring pro, I was exercising a horse for a not-so-good ammy rider. I am not handsy (in fact, I’m whatever the opposite of it is, not always good, but I lean toward too soft a rein at times). Rider was handsy. Her horse was GREAT for me, but when she got back on, her horse no longer appreciated the ride that her owner was giving her and began to complain.
Thankfully, I had the opportunity to ride with her normal trainer (who was about an hour away, so we only trained with her occasionally) and I will never forget her telling me that if I was going to exercise said amateur’s horse, I needed to ride like said amateur.
It was a practicality. Horse needed to tolerate a very different ride than I was giving it, and unfortunately when given a less “handsy” ride, horse stopped tolerating the ride it’s owner gave it.
I don’t know that a trainer picks one philosophy or the other and sticks with it so much as it’s a practical consideration. Some riders are just never going to be talented enough, and some don’t ever really want to put in the work to become so. But they still want to show, and they still want to win. The trainer then has to walk a fine line of training the horse and the person in a way that will work.
I think when you think of it as “this horse needs to perform regardless of the rider’s error”, it leaves more room for abuse to occur. But I do understand why trainers might think that the philosophy is valid (not necessarily abuse, but the philosophy that the horse must make up for the rider’s error).
I’d prefer my horses to tell me when I’ve screwed up, but I hope that they do so in a non-dangerous way. It’s hard when one overreacts to a screwup.