Please explain why sending a brief text is any more distracting than, for example, re-programming a route in the GPS, looking at a chart or approach plate and setting up an ILS with frequency selection with navaid identification etc. All quite accepted distractions from simply looking out of the screen or at the panel monitoring instruments and any warnings - the serious of which tend to be audio anyway in the A109.
Because all of the things that you mention are directly connected with the conduct of the flight, physically and mentally. In this case, the Commander of the aircraft's mind was on how to deal with his customer when it should have been on selecting Redhill in the GPS, climbing to 2400" and heading about 210 degrees.