Jesus Christ! That's really scary!
The thing is:
The police inspected, at first, the wrong seat. That's why they didn't find any mechanical problem. Then, they realized the girl was seating in an out-of-order seat and took a new look at the ride.
That restraint used to be lock all the time. For some reason, that day, it was unlocked. So the operator called his supervisor and said "the forbidden seat is unlocked" to wich, the supervisor replyied "ok, I'll the maintenance. Keep the ride running". (this is on the police information, I'm not making it up).
Well, if the operator new that seat was forbidden and maintenance was on the way, why the hell, did he let the girl seat overthere?
That is the question that must be answered.