I think the key difference lies in the effect of two (related) areas: (1) organized behavior; (2) coercion. As another poster said, what the WT does is "organized shunning," a very different thing than someone deciding on their own to avoid someone. Context is key. The WT presents itself as God's organization and, as we all know, uses the name "Jehovah" to indicate where direction is coming from, as in--"We want to do what Jehovah requires," etc. when in actuality, we could substitute WTBTS for "Jehovah" in that previous sentence. This imbues them with the veneer of divine authority and for believers, that is everything. So when publications indicate that parents should not even respond to a text message from a DF'd child, Witnesses understand that as a divine mandate and, what's more, see obedience as part of the "us vs. them" scenario. Consider how different it would be if the Society truly presented it as a conscious matter, offering scenarios where one person might decide to shun a DF'd child but another person continued association, both of which were presented as justifiable based on the individual's conscience. That would change the situation entirely!
Related to this is coercion. As another poster pointed out, if you do an act because a gun is pointed at you, that is not seen as a voluntary act. Under the law, contracts signed under duress are unenforceable. Similarly, disfellowshipping is a tremendous and significant method of coercion, if your entire support system/family is at risk, and so in no way can it be said that Witnesses can freely choose to have or refuse a transfusion. As long as the threat of organized shunning is a real risk, free choice is not available. Choice may be available but it is a coerced choice made under duress (the pointed gun) and so is not free (and therefore true) choice. It is as simple as that.