DHC6 Tropics
When I deadhead I would say that the majority of the time flight attendants become complacent during these situations and do not enforce the fasten seatbelt protocol.
This is a very sharp observation and I see this regularly, too.
The unfortunate lack of intervention reinforces the behaviour, which is wrong, when negative reinforcement* would make much more sense. (*telling pax that if they wish to avoid being injured, they should remain strapped in.)
Regarding being strapped in for hours, I once endured a flight from BOS to LHR with the belt sign on for the entire 6.5 hours! Therere may have been a very good reason for it, but I don't recall a single bump apart from the landing gear contacting terra firma at London.
People just ignored the belt sign after an hour or two, which is not conducive to flight safety.