I'm late to this thread but if this is a sincere question, there is a great article on the Huffington Post called "When Organized Religion becomes a cult." At the beginning of the article it states, "The distinction between cult and religion lies squarely in how those leaving or those wanting to leave are treated." I think that is your reason right there. If it walks like a duck...
Interestingly enough, a few days before the article on JW.ORG regarding whether they are a cult or not was posted, the Huffington Post tweeted the link for the article that I mentioned above. Could have been a coincidence, but who knows?