The tips of the blades of these drones can be sharp and they rotate fast.
Blade tips slicing into a person's face could cause a particularly severe injury to a person's eyes.
Untrained people fly them. Trained people can make mistakes. A pilot might loose control for no fault of their own.
Requiring drones to be fitted with guards around the rotor blades wouldn't solve every potential problem, but it seems like a simple method to address this particular threat.
An cheaper alternative would be to have a rotor blade cast with a ring connecting the blade tips. This ring would be a spinning surface that could cause a burn against the skin, but better than slicing blade tips.