Let's be real here, they were stopped because they were black.  The guy was tased because the he had the audacity to ask why he was being stopped.  Cops don't want anybody to question them really ever and particularly when they can't give them an answer.  Like in this case they couldn't tell the guy that they stopped him because he was black so they tased him to show him who was boss and then added insult to injury by then charging him with a misdemeanor.