I believe it is a risk assessment by the authority. They have two fully trained pilots up front to take over in CAT I and better conditions, but believe that may not always be enough in worse conditions and therefore want to avoid any possible risk of interference.
PEDs in flight mode still emit electromagnetic interference, and it is simply impossible to test every possible combination of PEDs (current, past and future ones, working correctly and malfunctioning) and aircraft wear and tear to be absolutely sure that nothing can happen. As bureaucrats everywhere, those from our CAAs or airlines apparently want to be more than sure that they are not responsible for anything, therefore they introduced that song and dance to make sure they cannot be blamed.
The company iPads in our cockpit are actually approved after they have been tested, our private iPads and smartphones fall under the same PED ban as the passenger devices. iPad testing is an ongoing process and every software (update) has to be tested before we can install it, which in the case of major iOS releases can take up to three months.