The reaction of destroying the camera is justified by Granddaddy Cain because he recognizes by the moaning sounds of Granny that the men have intruded upon their property and upset his wife with their exploitative intentions, already demonstrated to him with their filming of the death of the hawks.
Granddaddy Cain's actions are expressive of the title of Bambara's story: "Blues Ain't No Mockingbird"--feelings are not to be mocked; they are genuine. Granny and her family are not to be treated as some "typical" case without consideration for their family and their personal property and possessions. The two men from the county offices have trespassed upon the land that belongs to the Cains. So, when Granny expresses her displeasure that they are present, the men should have departed and respected her privacy. This pride in possession is underscored by Granddaddy as he pointedly states to the cameramen, "This is our own place."