It's all about probabilities. The chances of an event during the small amount of time you are in the toilet are very small, hence it is an acceptable risk.
Most of the injuries on this flight could have been avoided, except for the poor old cabin crew and the people who were hit by the falling contents of overhead lockers.
All of the cabin crew were hurt in varying degrees, but it is their job to be mobile in the cabin thus increasing their risk. Just about every passenger who was injured didn't need to be. FASTEN YOUR SEATBELTS like we tell you!