Because folks are assuming that an independant warhorse will pursue foes or go where the rider wants it to. In my game, if someone lets their warhorse act independently, it will attack those who have already harmed it, intimidate those who come near it, but will otherwise avoid conflict. Mount trained to be willing to deal with combat does not equal attack dog!
I agree with your interpretation. I have independent war mounts act based on their instincts. This generally means that if they or their rider are being attacked in melee, they will strike out at whoever makes the most sense to them to strike out. Since they are animals, it's sometimes best to let the dice decide who that will be. It's still usually worth it once you get into melee to let your steed do that, because them attacking someone is better than attacking no one.
What I don't have independent unintelligent steeds do is go where you want them to or attack who you want them to. I allow their riders to make an Animal Handling check to attempt to give them general directions (and don't charge them an action for that), but it might not work, or be less precise then you intended it. If the rider wants total control, the mount needs to stop fighting and just let the rider control everything he does.
Even if you don't allow the sort of Animal Handling I do, the rider still has a choice--use the horse as a motorcycle or as a blind hoof attack machine. Mr. Crawford is interpreting a spell that says:
"Your steed serves you as a mount, both in combat and out,
and you have an instinctive bond with it that allows you to fight as a seamless unit."
...as meaning that your Int 6 horse that you can communicate with not only doesn't gain the ability to follow your orders while fighting independently, but it loses the independent blind hoof machine attack option
entirely. As opposed to a horse you bought at the stable, this one
only has the motorcycle option.
So I say, really?
Part of me wonders if someone forgot that the normal mount rules allow an unintelligent mount to act independently or controlled, at the rider's option.