[fotbr]

They're looking at it wrong. The signs are operable by the flight crew.

Given: Aircraft power is on, signs are on. Aircraft power is off, signs are off. Aircraft power is operable by the flight crew.

Therefore: Signs are operable by the flight crew.

In addition to the aircraft main power system(s), there's almost certainly a circuit-breaker that would turn the signs off. It might turn other things off as well, and circuit-breakers are not generally designed to be used regularly as switches, but there's a circuit breaker in reach of the crew that would turn those signs off.