As already pointed out, there is only so much we can explain in the briefing and lets face it, how many passengers actually listen anyway? It's got to come down to trust in the crew. And that brings me to what, I feel, may be a contributing factor in the lack of trust passengers seem to have in the crew.
I'm going to go out on a limb here and no doubt offend a lot of people but, I feel you should have to be a certain age before being able to go for promotion to Senior Crew. I am a Purser for a major airline, and most of the time, it's a fairly straightforward job. However, when it's all gone wrong, I have had to draw on all of my flying and life experience. I have dealt with medical emergencies, technical emergencies, irate passengers etc. There's no way I would have been able to do this successfully at 21. No matter how mature you are, at 21, you do not have the "natural authority" needed to keep control of a sometimes, volatile situation. That's something that can only ever come with age.
This is not against the younger crew, as I work with some exceptionally good crew that I would trust with my life, but I know them. Passengers don't. Passengers can only go on their first impressions. Unfair but true.
For the record, I'm in no way, trying to defend the supposed actions of the passengers on the BMI flight. I'm just guessing at one of the possible reasons that these types of incidences appear to be happening.
Jsl
Hat...coat...