I have had many flights where the captain has said he/she is leaving the seatbelt sign on as a reminder to keep seatbelts fastened while seated. If and when turbulence hits, the fastened seatbelt sign is then toggled off/on to activate the chime, and then a separate announcement is made to ask people to return to their seats.

Given the reported number of situations where unexpected turbulence has caused injury (and the high profile nature of anything aviation related), I can see why U.S. airlines take this approach.