mcgoo - agreed. Especially as Trans-States Airlines, who operate the flight on behalf of United, use Embraer ERJ-145s on that route - at 50 seats, not the biggest aircraft you can fly in.
Reference to the debate about arming doors and slides - the 145 has a drop-down door complete with airstairs, so a moot point.
As for security - they are damned if they do and damned if they don't!
What would have been the outcry if they had just escorted the lady to the terminal and sent her on her way, then some device had later been found on the aircraft?
After all, passengers don't usually get left on parked-up aircraft - er, apart from the blind lady a week ago, who was forgotten after being asked to remain seated until the other passengers had disembarked. That was United too - but would have been a main-line A320, not a small feeder.