This incident is just the risk of a service orientated airline. You get stuff done, no seatbelt sign on when its not necessary, and the cabin crew can stay in the cabin when its not too bad. Here they hit some unexpected turbulence, we must suppose, I know KLM from within and know our policies. This is just the risk, I personally love this policy when flying as a passenger as opposed to the US airlines, with the fasten seat-belt sign on during the whole flight, just covering their asses. Nobody will care about the fasten seat-belt sign when its on the whole time, so eventually this creates a potentially more dangerous situation, because people dont care about it anymore when there actually is some substantial turbulence (ahead).
Best regards, fk