Most European Airlines mandate that you fasten your seat belt whenever seated, regardless of whether the seatbelt sign is illuminated. The rest "strongly recommend it." Maybe this would be the answer for US carriers to avoid liability issues. Agree that the main issue is whether or not it's safe to use the bathroom or get something out of your carry on or go get a drink or snack. It's not enough to make me switch carriers but it's certainly annoying and I often ignore it when it remains on for more than 10 minutes with no sign of turbulence.