At the end of the day, if the turbulence is bad enough that the punters should be strapped in, then it's also bad enough that the f/a's should be strapped in also. Their safety is no less important than that of the pax.
And ambiguous policies, e.g. signs on means punters must strap in but f/a's can keep working unless told otherwise just leads to confusion. The pax see the f/a's walking around so up they get too! Next thing you the airline is having its ar$e sued because granny's head went through the overhead bin... "but the f/a's were walking around, your honour, the implicit message was that it was ok to get up".....
It's all about being unambigious, protecting everyone on board and protecting the airline from being sued.
The QF policy works. Get over it!