Lufthansa requires passengers to wear seatbelts AT ALL TIMES when they're seated
Many other airlines encourage this too.
However, the emphasis is "when seated". Not everyone might be seated at a given moment for a variety of valid reasons. This may have been the case here. We don't know, we weren't there.
I keep my seat belt fasten at all times
when seated. I often reflect how that's not going to save me if we happen to hit unexpected turbulence as I make my way to the lavatory. Sod's Law!