It actually isn't safe for you to walk around, however since you are an employee and not a passenger you are asked to assume a certain level of risk that the pax are not. Ever notice the the FD crew are ALWAYS buckled in with at least a lap belt? You never know when the aircraft is going to hit unexpected turbulence.
Since the crews job is to keep the pax safe as possible if there is a known possibility of turbulence the signs go on. The cabin crew are asked to assume a certain level of risk in remaining standing and accomplishing their duties, which is why so often its cabin crew that get hurt in turbulence related incidents.
If the FD expects to run into more than light chop they will almost always advise the Purser to seat the crew and suspend service in order to protect them as well. problem is we don't always know when that is going to be.