Yeah also because departure and landings are the most critical times for aviation safety. If you're jabbering away on your mobile phone then you are not paying attention to the stewardess who is demonstrating important safety features that could save your life one day. Also if the aircraft encounters difficulty and there is a real emergency, you don't want a bunch of people enslaved to their electronic masters rather than having situational awareness and responding and obeying the flight safety crew (the crew is there to ensure your safety, not to feed you Coke and pretzels.)