Let's try to summarise the key points so far.....
Do CC request the FSB light for reasons other than safety - YES & NO
Can you have one rule for allowing people to go to the loo when the FSB light is on - YES & NO
Can CC make a judgement about whether it is safe for a pax to go to the loo - YES & NO
These are from posts by crew
Van Horck gets pilloried for asking for communication by Boarding Pass, who finds it difficult to communicate with passengers, yet can apparently receive PIREPS to decide whether it's safe to walk about in turb. That's definitely paranormal.
Then we have the classic industry attitude to educating the pax .... "In regards to the suggested movies showing the consequenses of turbulence and not wearing seat belts, I think we'd have even more pax scared of flying. Statistics show 40% don't like flying as it is, so that would take the percentage higher and possibly lose us more pax in the process."
In conclusion, if
- the pax are not fully educated to the dangers of the environment
- the crew have cognitive bias about communicating (as demonstrated by some here)
- the people paid to do the job don't agree about the use of the lights
Then don't be surprised if you have fun and games with some of your guests.
I also put it to you that the people who walk around in mod turb and choose to leave their belts undone are usually not PPLs or FFs, despite the predisposition of some around here who choose to sneer.