Interesting question Simon.
I haven't read the entire thread and am only jumping in (late in the conversation, I know) addressing the points in the OP.
It's important for us to recognize that what is right or wrong is not the same as what is legal or illegal.
Also, that any individual has the right to shun anyone else is surely not the same as institutionalized, coerced shunning as practiced by cults such as Jehovah's Witnesses and many others.
If you don't want to talk to me, then I guess that's your right. But if you won't talk to me because someone else told you to, then that's just messed up: you're in a cult!