I think the cockpit crew just forgets to turn it off. When they hit turbulence or get a report, that triggers it to turn on. There isn't anything to trigger them to turn it off other than passing to the next sector and getting a new turbulence report saying its clear. Also, I'm sure the err on the side of caution, your chance of getting injured while belted in is much lower than not belted in.
However, pilots should know better than anyone about the danger of nuisance alarms. If you keep it on the entire flight. Passengers eventually need to get up to use the lav and now you have no way of gauging how safe it is because the pilot took away the only indicator. It's equivalent to keeping it off the entire flight