Scratching my head over this thread. Yesterday's DEN-PIT flight was typical in my view...seatbelt light went off at 10k and was turned back on once for maybe 10 minutes when we hit bumps near Chicago. If the light IS on in smooth air there's never been any issue getting up to use the facilities...just don't ask if it's OK.
Maybe some flight crews err on the side of caution and/or there may be liability issues. Have any US airlines been sued because there were turbulence-related injuries when the seatbelt sign was off?