Here's why I think they are doing it - it removes the horse barbs until they are researched by at least one nearby Civ; or maybe even by the player themselves.
So it solves the issue of some players nightmares in horse barbs right out of the gate; without changing much else about them. I think it's a finely balanced smart move myself.
Why not just require a civ to have learned Horseback Riding before the barbs get horses, which is in any case consistent with the way barbs get every other unit (I think - I don't think they get swordsmen with Bronze Working)?
I imagine the following conversation in barbarian camps:
Barbarian 1: Have you heard? The Scythians have discovered a powerful new weapon they call the horse.
Barbarian 2: Horses? I thought they were just a myth. What do they look like?
Barbarian 1: See that unicorn over there? Like that, only without the horn.
Barbarian 2: How do they use that as a weapon?
Barbarian 1: I'm a bit hazy on that, actually. I think they use its strength to help work the land and increase productivity. But they might just melt them down into raw materials.
Barbarian 2: Seems a lot less effort just to jump on one and charge the enemy with it.
Barbarian 1: Yeah, I thought so too. Hurry up, we've probably only got a couple of centuries before they come up with the idea as well.