This umbrella holder is not designed to hold an umbrella while the cart is in motion, indeed for many users it will be too low to achieve this. It is designed as a place to put an umbrella while you take a shot and the cart is stationary.
This is a safety precaution designed to remove the chance of the cart becoming unstable when used in windy conditions, as a remote control or follow machine could be a distance away from the user when in motion.