Tiller steering is dangerous, that's why. I know of someone who did that to a lawnmower and had it ripped out of his hand only to swing around and split his hand open.
Any non-power assisted system can hit you back hard in the right conditions. If something is regularly a pain in the butt to operate then it's going to go away, just like manual steering, synchronized transmissions, non-power assisted brakes and hand crank starters. No need for a law.
I think you meant "non-synchronized transmissions".