If that is the reason, then it's stupid, and i'm siding with Dean Barnett. Seatbelts have been mandatory in Australia since 1972 so people shouldn't be having too many difficulties with them.
Operators check seatbelts prior to dispatch, so an unsecured seatbelt would be identified, and the train quickly reset, and restraints re-checked. The amount of time this takes, on the rare occasion that someone is incapable of operating a seatbelt would be far less than the additional time it adds to every dispatch for an operator to do it up for each and every rider.
Then again airlines still have to show people how to operate seatbelts, so maybe there are too many lowest common denominators at our parks these days.