A quick comment from an SLF...
Every time there are turbulence injuries the passengers are described as stupid and, in many cases, I totally agree. If the seatbelt lights are on, you stay buckled in.
However, from reading, sometimes the worst incidents happen unexpectedly without prior warning. I always travel with the seatbelt fastened (force of habit) but like every other piece of SLF, sometimes have to make the trip to the lavatories. On any plane carrying several hundred people, even if every seated passenger is buckled in, percentages say there will always be some on the way to/from/in the toilets, not to mention the cabin crew going about their duties.
So...no sympathy with passengers who ignore turbulence warnings and seat belt lights...but it's not always stupidity.