I've only ever shunned people if they have repeatedly made me feel uncomfortable, so it's very hard for me to understand the motives behind what the JWs do other than fear of getting caught and being shunned oneself.
One thing that has always puzzled me is why more JWs don't communicate to their DF'd friends and family in secret. In today's computer saturated environment of anonymous web-based email and PGP encryption, getting away with it would be trivially easy and yet not one of my friends has agreed to communicate with me this way. Are they afraid that Jehovah will tell on them to the elders? I would think that the very naughtiness of such an act would prove attractive and actually deepen friendships on the principle that the relationship has now become totally private instead of being mediated through the bOrg. Also, it would be a way of rebelling against the elders without taking on any real risk.
I guess I just have no grasp of human psychology.