Flight attendant power?

Flight attendants don’t control the seatbelt sign.

The flight attendants do NOT control this - it is controlled from the flight deck.

The flight attendants aren't the ones who control the seat belt lights.

You really are out of touch with the job flight crew do when not looking after YOU..

The Pilot decides when and if the sign is turned on.

OF COURSE the crew might request it be turned on for a variety of reasons.

 I'm guessing they do 

Of course in stores you shop the cashiers just close their line to stand around too?

As there are about a Million flights per week the crew on ONE might to that.

the pilots in the cockpit control

the seat belt sign not the flight

attendants

The captain controls the seat belt status.

Flight attendants often still are going about their jobs when it is on.

Flight attendants don't control the seat belt sign in the passenger cabin. That's up to the pilots.

No. It's purely a safety matter. It pays to keep the belt on at all times anyway as sudden turbulence can whack you onto the ceiling.