I am a HUGE advocate of freedom of speech. However, when that freedom interferes with your ability to do your job, I would say that you can be fired at will.
Now, many people have said that unless he actually does an act of racism on the job, he should not be fired. However, I tell you as a former criminal defense attorney, that I would bring up his racists comments every time I represented a minority client who was arrested by him. It would be my duty to examine whether there was actual probable cause or whether he was just acting on prejudice.
Also, think about what would happen if he had to shoot a minority while on the job. The press would be all over the statements on the KKK site and the city could even be thrown into a riot over it.
One more thing. Now that the press has picked up the story, how do you think minorities are going to react when he goes to ask them questions, pull them over, arrest them? If I were in that position, I don't know if I would pull over for this weirdo.
Nah, I think he should know that posting on a KKK site would get him in hot water and is jeapordizing his effectiveness as a cop. Cops are entrusted to serve and protect. He can't be trusted in this position after exercising such bad judgment. He should eitehr be fired or given a harmless desk job.