cell phones and planes

why do we turn off our cell phones and all other electronics during take off and landing? regardless of the exact reason, it will interfere yadda yadda yadda, we all obey. first, we are going to fly thousands of feet above the ground and anything to assure a safe flight we will do.
in this current day and age there are however, electronics/iphones/bluetooths with an "airport mode" switch. this enables the phone to remain on during the flight; however, this does NOT mean it can remain on during take off and landing. honestly, it still interferes with radio connection. meaning, without clear radio connection on the ground how is the plane supposed to maneuver through the airport??
real story: i sat by a very nice, friendly woman on my flight from chicago to raleigh. chicago is a huge airport and we coasted through the maze of planes for about ten minutes before we took off. i noticed this woman kept her iphone on during take off. i wasn't sure so i didn't say anything. as we were descending into raleigh she was using her phone and using whatever application for entainment the stewardess came over. he told her to turn off the iphone and she proceeded to simply darken the screen to black. she did not turn it off. she merely kept the screen black to "fool" us all.
i understand these devices are capable to remain on furing the flight, but not during landing and take off; however, for her own ease as well as my own, why would anyone risk a complication with a flight full of a couple hundred people?? i mean really, i think she has rebellious tendency's which she never really let loose during her adolescent years.