First, there is not a bigger advocate for firearms on this forum than I.
Most every police agency in the US that uses the Tazer has a demonstration/training program where the gun is used on those attending the class. I would be surprised to learn that this was not the case for this officer. Why didn't he have his buddy check it out on him? Could it be he is just a coward wimp and and shot the cow because he is afraid of a few volts of electricity?
Ask any Law Enforcement officer and I am sure they will agree that one of the greatest assets to maintaining law and order is a good public image. Stupid tricks like this do foster a good public image. Using the justification that you want to know how the tool works does not make sense either, what next, do you support checking out your .40 auto on the neighbors dog to see if it works? It is not a question of being a " PETA idiot puppet," but rather a question of common sense and logic as applied to, most likely, government owned equipment, and the expected respectable performance of a public servant.