Passengers must follow the instructions of the crew. There is no excuse for not doing so. But today people no longer will follow instructions if they don't see anything in it for them, particularly when they think its a stupid rule. "They use iPads in the cockpit, why can't I use my android here?"
On the other hand FAs are not policemen/women (or class monitors). I don't see their role as including the seizing of passenger's (expensive) property. If you want to cause a disruption this will do it. That individual may be a jerk, but s/he could also be a commercially important customer for that airline. (or a retired bum like me
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It is almost impossible to tell if a device is in airplane mode even at a short distance. Yes, there's a little icon but every device is different. I bet you can find an app that will toggle that little icon. On my iPad I can save web pages to a reading list intended to allow you to read it when off-line. It looks like I accessed the net, but I didn't.
I think the industry, and the regulators, need to face reality. These devices will be inadvertently, and intentionally, left on and people will attempt to use them in all phases of the flight. Starting from there they need to consider what needs to be done to ensure safety. I'm not going to suggest what that may be. However I will note that the current rule and procedures are not accomplishing the stated goal.