Why do passengers question every single thing they are requested to do on board? And why are flight crew oblivious to the reasons if the applicable FAR
isn't spelled out explicitly?
Is there a FAR
that states all wings must be attached during takeoff and landing? Well not explicitly!
Check out FAR
121.310, 121.291 and FAR
25.811 and FAR
25 Appendix J with respect to emergency exits. After all that research, consider that it isn't all that cut and dry how or why you are told to comply with certain requirements during your flight. Some aircraft may require special consideration to meet the evacuation requirements.
For example, there aren't any regulations directly requiring table trays to be stowed during takeoff and landing, but there are regulations regarding evacuation and aisle width during emergencies, which tend to occur during takeoff or landing.
All this doesn't mean that every aircraft is the same or that sometimes you won't be told to position the window shades for courtesy sake. But all you have to do as a passenger is listen to the flight crew. Is that too much to ask?