BOAC, in your quote I can not find any rule about the fasten seatbelt sign. During the passenger safety briefing, it is ofcourse emphasized that keeping the seatbelt on while seated is much safer. However the sign may be switched on or off at discretion of the PIC.
AFAIK the EU OPS does not have any regulation about this and if they say that the sign should be on during the entire flight, it's nonsense in my opinion. The sign is switched off mainly to let the passengers make use of the lavatories whenever they want to. And it will be switched on when the PIC deems it necessary because of (expected) turbulence. If the sign is constantly on, the cockpit will constantly be bothered by the cabin crew 'ding dong' "there is a pax that needs to use the toilet, can he/she go?"...
edit after rereading: BOAC I think you missed the word 'sign' in the initial post...