The phone I have, asks if I want to enter flight mode right at startup, but when on flights, I can't be bothered using it.
I do have some sympathy with flight crew, as has been pointed out above, it is hard to know every phone model, so maybe easier to adopt a switch off rule, rather than try to assess each one.
Just as an aside, as the original poster was taking pics, I was recently on an AA flight to La Guardia, which swooped low over Manahtten on final approach. Cabin crew asked a kid who was taking a pic with a digital camera to switch it off, as it was an 'electronic item'! I guess all airlines approach this kind of thing differently.