Although this doesnt justify someone being injured atleast in that circumstance the company will take full responsibility for the injury and any sick time/ possible insurance claim related to the injury as they had given them the opportunity to be released from their seats.
The same risk applies to performing a safety demonstration whilst the aircraft is in taxi. I have fallen over once, I wasnt injured (a part from my dignity!) but I did write a report and the company backed me completely as I had permission from the command to be out of my seat, however an incident once occured where a passenger was up during taxi in and fell over hurting his arm. The arrival PA had already been made advising pax to remain seated until the seatbelt sign was switched off and the passenger had no leg to stand on as far as making a claim against the airline as he had disobeyed Civil Aviation Law.
It must be very frustrating for passengers though, they havent been exposed to everything most of us have in training and being out online and all they see is us getting up and possibly having no regard for the rules they have to follow.