There are times when we are supposed to be up when the seatbelt sign is on. If there is severe turbulence, yes, we should be seated and belted in. But, for instance, (and I'm speaking for AA policy, I don't know about US or other airlines) as soon as the captain signals the end of sterile cockpit at 10,000 feet, we are supposed to get up and begin preparation for our service. The seatbelt sign usually stays on until we reach cruising altitude; so, passengers are supposed to stay seated. There is no FAR that says flight attendants have to be seated if the seatbelt sign is on. There is, for another instance, an FAR that says once the captain tells us to prepare for landing, as soon as we get the cabin picked up and do our safety checks we are supposed to take our jumpseats, belt in, and stay there until after landing. Granted not all f/as do this, but it's one of the few instances where there are specific FAR instructions about us being seated or not.
I once asked an FAA inspector what we were supposed to do about passengers ignoring the seatbelt sign. He told me that "whatever you do, do NOT give them even implied permission to be up." He specifically said that such statements as "You are up at your own risk" or "Just be careful" have been interpreted by the courts as implied permission to violate the FAR. He suggested that when they get up (or if they ask you, "Can I go to the restroom?") just say, "The seatbelt sign is on." If they have asked you if they can get up, just say, "the seatbelt sign is on." If they ask you again, just say "The seatbelt sign is on." If they persist, say "Sir/Ma'am, what I can legally tell you is the seatbelt sign is on." If they then choose to ignore the sign, you and the airline are free of liability.
The Continental Express case last year is a perfect example. A woman got up during severe turbulence despite a PA from the captain for the flight attendant to take her jumpseat, and a PA from both the captain and the flight attendant for all passengers to remain seated. The woman insisted on her right to go the lav. While she was in the lav, they hit an air pocket. She flew up, broke her neck on the lav ceiling, then broke her back on the toilet when she came down. She tried to sue CoEx, and the court threw it out saying she was out of her seat in violation of Federal law after clear warning from the crew.