This seems to be the case in all US airlines, and the opposite on most foreign carriers. (The difference being, on foreign carriers they take the seatbelt sign quite seriously.) You are correct that this means in practice, most people just ignore the seatbelt sign once the plane takes off.
This. Its the overly litigious nature of the US society. Dont want to be the one in trouble cause you hit bad turbulence and people werent strapped in so just easier to leave it on.
I MUCH appreciate the way international carriers treat the seatbelt sign cause you know that if it comes on it actually means something. They use it properly.