Drives me crazy, as do those who cannot obey the seatbelt signs. Many people just do not respect the environment they are in or the crew that work there. I've seen people standing up trying to get luggage out of the overhead lockers while we're still doing about 60kts on the runway. My airline is very good at ordering people, if necessary, to follow the crew's lawful instructions. After that, anything they do is at their own risk. The aircraft has very powerful brakes, so you really should keep your belt on!