Just because you (and others here) aren't aware of something doesn't mean it doesn't happen.

As some here may know, once upon a time I was a UA F/A. It was definitely common for pilots to forget to turn off the sign, or forget to let us know it was now safe to get up after a "F/As be seated" PA. We would make a simple call to the flight deck, which passengers wouldn't know about unless they happened to be next to the F/A who was making the call at that moment, to ask if they still wanted the passengers or F/As to remain seated. For the sign, I would usually call when it was on long enough in calm air that passengers were getting antsy and using the restroom more frequently despite the sign because they are human (and believe me, I hated pointing out the sign is on and getting the "but I have to pee" response even more than passengers hate hearing it), which I definitely sympathize(d) with because I have a weak bladder myself. After the a "F/As remain seated" announcement I would call if it was on long enough in calm air but we had to start the service. Sometimes the pilots did want us or the passengers to remain seated, but usually they had just forgotten (because they are human too!), would thank us for the reminder and immediately turn the sign off.

Now back to the regularly-scheduled conspiracy theories.