People ignoring the seat belt sign is simply a fact of flying in the US, IMHO primarily because pilots in this country tend to be overly conservative with the seat belt sign. I've been on a lot of international flights where the seatbelt sign gets turned on, there is about 10 minutes of turbulence, and then the sign stays on for another two hours. At a certain point, people need to use the lavs.
When I fly on foreign carriers, I'm always surprised at (1) how quickly the seat belt sign gets turned off after take-off, (2) how long they wait before turning the sign on before landing (often just 10-15 min before landing) and (3) the level of turbulence it actually takes for the sign to go on.