It's not about the right to shun. It's about the wrong of teaching others to shun a particular group of people.
To see what I'm getting at, replace shuning with hating. Do you have the right to hate an individual? Yes. Do you have the right to publish information to instruct the public to hate a particular group of people? In many countries that's a crime called hate speech.
By instructing their members to shun those who disassociate from the JW religion - on the threat of being shunned themselves if they fail to comply - Watchtower is engaging in immoral behavior that is, in my opinion, not much less criminal than hate speech.