Not only are those little girls, but:
1. She's trying to hold on so the horse doesn't run off.
2. The horse is not dead.
3. No one's about to hack her in half with an axe if she sits there.
You'd better believe you'd be rolling and getting your sword ready. Contrary to having your face caved in with a heavy maul and then somehow rolling around totally concious and care free. You're actually more likely to be able to think clearly and react while full of adrenaline and skidding down with a horse... you may have the occasional broken limb / broken rib (concussion / broken back if you're unlucky) .
There's a chance you'd get a leg trapped under and I'm even happy for that to be a random % based "yeah no rolling for you sucker". But right now, it's easy mode killing a downed horse to the point that every assclown is swinging so wildly for the free kill you're likely to cause a team kill just by dying.
The natural reaction of a person when falling from something moving is to keep rolling with the momentum and direct it - This limits the damage. People are quite hardy in falls and can fall out of moving cars, rolling like lunatics and walk away with bruises. It's the dead impacts and skids that cause the real damage ripping skin off and breaking bones in half when you hit trees.