Are we missing the point? The seat belt sign was ON. Why were the SLF not strapped in? Probably for the same reason they don't listen to the passenger brief!
Crew injuries might be explainable, but the SLF injuries were avoidable, but probably deserved - am I being hard on them? Maybe, but too many people believe that they can always do as they want with no consequences. Remember the one on the wing in the Hudson with his life jacket on backwards?