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The reason that you are having difficulty locating a specific regulatory reference for the armrest is that there isn't one (unless your local CAA has it written into their regulations). However, all of the reasons mentioned above are valid ones for an operator to consider having the requirement in place.
As well, the seat back is designed to be an energy absorption device for the passenger seated behind -- as that person comes into contact with it during a sudden deceleration, the seat back absorbs the force of the person impacting it and transfers that energy through to the seat base.
If the armrest is left up, it is possible that it could cause injury during an impact sequence if there are lateral forces involved. However, the seat manufacturer must take this into account during design and testing to demonstrate that the passenger will not suffer serious injury in an emergency landing as a result of the inertia forces specified in the design standards.