Originally Posted by
thebat
That happened in the US too. It was the rule until 9/11. Has everyone forgotten that already?
I remember well the days before 9/11. I was more of a frequent flyer then than now.
It was never the "rule" in the US that people hopped up when the plane landed. Then, as now, most people waited for the ding to hop up. It was likely more frequent then than now that people undid their seatbelts on the QT while awaiting the ding. Then as now it violated FARs for people to get up prior to the ding. If a FAA inspector had been on board, and the FAs did not instruct the one getting up to sit down, the airline would have been fined.
At least in the US, the technical answer to the OP's question is that FARs say so. Others have addressed why the FARs say so.