You would be surprised at the actual laws concerning filming overhead a country. The list of states that ban it is extraordianry. Pull you camera out on a Greek apron or overhead Greece or Turkey and you will probably be locked up. The cabin crew are made aware of many states where it is downright unlawful, and will not discuss it. If you persist, and somebody makes a report, trouble can come back on them. Therefore if you want to make a discussion of it in the air, and they get short of patience, then you really can't be surprised if they take you to task. They are not being bossy- they know the rules for most states and are not prepared to see you putting them at hazard of action taken officially.
It is not a discussion point, you do not have a leg to stand on. They speak with the authority of the Captain. If you were reported for persistently filming despite instruction, I would pass this on t the destination. You may have policemen with long droopy mustaches and big truncheons meeting the aeroplane with a lot of interest in what you have been filming. You would be surprised at how neurotic some states can be- ask the plane spotters locked up en masse in Greece!