Gravity supplies more than enough power to reach dangerous speeds even on unpowered devices. If all the governor did was remove applied power once walking speed was reached, that would be enough to get kids to the top of a hill without effort, and then they’d go rocketing down to their doom.

The danger of these scooters is that they’re scooters: they’re unstable and unsafe at speed. Bicycles are much more stable and thus safer at those same speeds.