Earlier quoted context omitted.
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."
I just assumed all airlines did it.