In the end, it's the FAA that makes the rule that anything not deemed a "small handheld electronic device" must be stowed for takeoff and landing. It's also the FAA's call that the seatback pocket is not an appropriate stowage space.
It's a Flight Attendants job to prepare the cabin for an emergency evacuation for every takeoff and landing. Ultimately, that's what it's all about. People have come to take safe air travel for granted. I imagine the folks that died on the Russian airplane would have appreciated people not stopping to get their carry-on items from the overhead bins. I imagine people who were severely injured in unexpected turbulence wish they had left their seat belt buckled while they were seated. I bet people that have been injured while casually strolling to the bathroom in turbulence probably wish they had waited. It's an airplane. A metal tube, careening through the sky at 600 MPH - what could go wrong?