Why the shunning if someone leaves? Why are they working so hard to keep everyone in? Is it the money?
I do not believe it is the money. In our situation, we had quit giving money before we quit giving our time (meetings, etc). If anything, doubters are a financial drain on the Borg.
I believe the shunning policy has to do with "Information Control". (Let's review: It's a Cult!) The Powers That Be (GB, etc) know that if Doubters and Skeptics share their doubts and skepticism about the ridiculous doctrines (New Light, Old Light, Flashing Blinking Lights) with other JDubs, the doubting and skepticism will be "contagious" and result in many others who go about "whispering" their doubts to others, etc, etc before they finally walk out the door.
Starting out, some may be sincerely seeking the answers to their doubts, but as they dig deeper and discover that there are NO satisfying answers they come to realize that they have been scammed. The way to shut them up is to threaten them with shunning and the loss of all their "friends" and JW family.