Don't forget that if you are using a camera during takeoff and landing, then you are distracted away from the cabin environment. Thus in the event of something serious happening, you are likely to either a) not notice or b) keep using you camera (thinking of the fortune post event) putting yourself and therefore others seated near you at risk, rather than following crew instructions.
Thus, the crew (acting on the Captains authority) are issuing you a lawful, safety related, command.
I wonder if the Ryanair change of policy follows the incident at CIA?