Why does the voice from the police car respond “No profession”? in the story "The Pedestrian."

The voice from the police car says this because Leonard Mead has said that he is a writer.  The car says this as if it is talking to itself.

In my opinion, this is a commentary by Bradbury on what this society is like.  The society is totally rigid and structured.  People are not allowed to do anything that is not like what everyone else does.  That is why Leonard Mead has been stopped by the police -- he is out walking when no one else would be.

In such a society, a writer would not do well -- no one has any imagination.  So the police car does not even recognize the idea that someone might write as a profession.