While it may not yet cross the street to legal issue, it is safe to call it an ethical issue. For instance, and I am speaking from experience, choosing not to associate with someone who has spiraled to a point of absolute danger to them selves and others. having to take somewhat of a "intervention" approach and not enable them through self destruction (which doesn't mean shunning either). Just not supporting it.
But when you consider the worst part of witness shunning its not really someone who breaks a witness law like fornications and gets DF'd but still believes fully in their membership to the cult, but rather the worst part would be anyone who simply says, I don't want to be a member any longer. This is where you see the true intentions of this cult practice. Censorship and vilification/demonization of anyone brazen enough to leave. You are saying you become a jehovah's witness you have forfeited freedom of religion in their eyes.
The damages cause to someone simply wanting to leave, should start to push the line from ethics to legality someday I hope.