The balance mechanism will keep them from falling backward - but interpret a heavy backward arch as "reverse fast" and a forward jerk as "damn the torpedoes, all ahead full" and a sideways flop as "spin wildly". So it's less a problem of them falling out or falling over, as one of them dashing around like a panicking bull, ramming things and people at 20mph.
Of course, there's nothing to stop the manufacturers programming in different modes, imagine speed limits, sports modes with higher torque and lower battery life, etc, etc. If some such "The operator is prone to seizures" mode were to exist, to require more prolonged sustained input from a user, or to move very slowly, after "double inputting" an intent, I can see this being quite safe, a "lean forwards twice to toggle moving forwards at 3mph" may well be a totally acceptable interface for some subset of people.
