I don't get this one either. I've never actually been on any lifts without one, but I've been happy to have used the safety at the time when an emergency stop once almost threw me out of the chair.
But most of all, I find it very strange that they are so rare in the US than here in Europe. I usually find your culture with warning signs on literally everything, and "sign-here-so-you-won't-sue-us-forms" for every other potentially hazardous activity more likely to have safety bars than the Laissez-faire attitude of say, the French.