Although I don't believe putting people in prison actually helps society apart from a number of extreme situations or people, it is also acutely true that certain jobs are invested with an enormous, almost ridiculous, degree of power over the regular citizen. Being a policeman is one of those jobs.
The kinds of words that got this person into trouble - whether on or off-duty - should be of great concern to the public and should never be overlooked or excused as long as they still perform their job. Policemen are people and people make mistakes, of course; but whilst some mistakes are excusable, this type should not be.
Should an individual with no significant power over others invested in them say the same things, I agree that they should not be jailed for saying them.