Why do flight attendants continue to stand, when the seatbelt sign is on?
I worked as a flight attendant for several years and I am still employed within the industry but in a different capacity, so I know a thing or two about this topic.

Flight Attendants (FAs) are strictly required to be seated during take-off and landings only and usually not during descent or climb, unlike passengers who are usually required to be seated and buckled. Then it depends on the local rules for how long they are supposed to stay there. In some airlines, the FAs must sit as long as the seatbelt sign is lit, while in others they can move about once the gears are up (you can hear this easily, as they make a loud sound when they are retracted). Other airlines might state FAs can move once the plane reaches a certain altitude and/or are given some sort of sign from the cockpit.

There is not one type of turbulence, but various classifications. Some of them are just light (small bounciness) and some are dangerous and can injure (or kill) people who are not seated and buckled. It is usually cabin crew who are the victims (I have personally been through this kind of turbulence). 

Usually, pilots will, during the pre-flight briefing, brief the cabin crew of any expected turbulence and will usually say something like: "seatbelts are for everybody" or "seatbelts are for passengers". If things during the flight end up different from the forecast that was given to the pilots prior to the flight, the pilots will call the cabin crew and inform them. They will tell the cabin crew whether to continue the service because it is a light turbulence, they may suggest only to serve cold beverages, or tell the FAs to sit tight and secure everything if approaching severe turbulence. 

This is basically the "secret" behind it. There is no magical training given to cabin crew on how to defeat the laws of physics and stand balanced while in turbulence; it is just the word from the pilots as they are the most knowledgeable about the weather conditions. The crew are only trained on types of turbulence and on how to secure the cabin and service carts quickly in case of sudden severe turbulence that was not detected in advance. In that case, they are told to hang on to anything and secure themselves.

Passengers, on the other hand, are required to be seated and buckled during all phases of the flight except cruise and when there is no turbulence. This is simply due to liability and not to add more complication in these critical times.