Speaking as a pax who fastens the belt so rapidly, I often have to undo it to let the other passengers board into A and B
, I feel I need to point out that people DO need to go the toilet.
Especially on longish flights, and in cattle class, queues build up at the loos as the journey nears its end. I should thing that having twenty or so people on their way/waiting/coming back from a toilet break would not be unusual in a 747. If each of those people hits the roof and bounces off one other person, you get up to the forty injuries level pretty quickly.
If the FSB chimes unexpectedly mid-air, it can take a while to struggle back to your seat, especially if you're going against a tide of returning trolleys.
All I'm saying is, don't be too quick to lambast pax for foolishly not fastening their seatbelts.