When the seatbelt sign is on at any time during the flight, the FAA requires that you remain in your seat with your seatbelt fastened. This is for your safety and teh safety of other passengers. If you are walking down the aisle and you fall on someone and hurt them as well as yourself, it is a big problem. We are put in a very bad position, We are required to tell you to be seated. We do not do it to be mean. At least I don't, but I wouldn't be doing my job if I was to neglect this aspect and rule.
Just ask me for a Depends ;-)
Just kidding! Merry Christmas to all!