Because she's a show-off and an idiot, and they figured it out after she'd been on it ten seconds.
The first thing she did after toodling around for a few dozen feet was to yell out how do I make this stop! -- in utter panic -- and then let go of the handlebars. I guess she thought it was like a lawnmower, since it looked like one. Let go of a (modern) lawnmower, and it stops.
Well, the scooter didn't stop, and when she started trying to balance on it (in panic) by her feet alone, it sensed her movement, and then it went into some kind of panic mode too. You could see the vertical post and handlebars doing a rapid back and forth oscillation, reminiscent of an Airbus taking off from JFK :) as the thing tried to jiggle around to keep her upright.
I think it was after that when the "shadow" started following her around, and there was a time or two that he had to grab her. She wanted to show off by lifting one foot, and the scooter of course sensed the imbalance, and tried to compensate, and she about went flying -- off it.
I could just imagine the sinking feeling in Kamen's gut as he watched that idiot push the thing to its limits and beyond while trying trying to be the center of attention. Can you imagine the bad press if she'd managed to run over more than one foot, or into someone, or into a brick wall, or, get thrown off it?
I expect that for any future demos they'll make damn sure that all "participants" get the riot act read to 'em before they're let near the things.