The light only means that having it fastened is mandatory. There are many reasons to keep it on during the entire flight, the most common of which is turbulence.
In Europe that's what seems to be a standard announcement. "The captain has switched off the fasten seatbelt sign, but we recommend you keep your seatbelt fastened at all times when seated, in case of sudden turbulence."
It’s the same with canadian and asian airlines. Is it not the case in the US?
I just assumed all airlines did it.
Last year I flew domestic U.S. on several airlines and the preflight safety briefing on all of them included a line that passengers should always keep their seat belt buckled for safety whenever seated (or something to that effect).
I didn't want to claim it was universal when I've mostly flown within Europe.