I was on UX flight 6968 form Sacramento to LAX on Sunday evening. At one point a passenger got up for something, either to get something out of the overhead or go to the bathroom, while the seat belt sign was on. FA then said over the PA that the seat belt sign was on, and she needed everyone to stay in their seats. Then she said she would appreciate our help in this matter becasue she can be fined by the FAA if someone reported that a passenger was standing. Has anyone else ever heard this before?