The Commander is given "Cabin Secure for Turbulence" which is different to "Cabin Secure for landing" ie the crew are not strapped in. Unless, as Britanniaboy stated, the Flight Crew tell us to sit down and strap in as well.
And to those non-cabin crew on here - have you tried to keep passengers in their seats when the seatbelt sign is on? Theres always some who insist on using the toilets no matter what we say!