As cabin crew...I must admit more often then not you feel the turbulence BEFORE the seatbelt sign is on. Im sure any pilot will admit that it is impossible to predict turbulence more often then not and they can only simply rely on reports from aircraft ahead for clear air turbulence.
As for the cabin being secure....well if you have 4 crew on in a cabin of 200 people it could take anything up to 5 minutes to ensure that all the toilets are unoccupied, babies out of bassinettes, passengers woken and strapped and all this while hoping the turbulence does not get any worse simply so the crew can safely carry out this check. But also on that note it can often prove impossible to "force" passengers to fasten their seatbelts (especially when they see crew walking about...IRONY!) and heaven forbid you get some gobby passengers trying to go to the loo "BUT ILL PEE IN MY SEAT"!!!! I hear that EVERYTIME the seatbelt sign is on for extended lengths of time.
Alas, passengers JUST DONT CARE about their own safety or the safety of those they will impact upon should turbulence become severe. It was slightly different for about a fortnight after the AF447 but alas the general travelling publics memory is short lived!