Because it shows everyone in the community how cruel, unloving, hateful and distorted the cult can be at a moment's notice
You are so right when you said that statement. My friends in my neighborhood have asked me so many questions as to why the JW's shun and what the JW's believe.
Most of them reply with: They can't believe that a religion teaches their members to shun family members because they choose not to be a JW's any longer. 'What religion teaches their members, not speak to their parents, children, siblings...that is sick." (That was a statement made to me, just yesterday by a 81 years old man who asked me about my family).
Dee