In fairness :-
A) I am also a shunner of many of my former friends.
B) I have made my son a shunner by "preventing" his association with kids that I feel are a bad influence on him.
What is the difference?
Well being realistic, I think we should shun people who are not a healthy influence. ( A recent headline in the papers, was of a lady who tried to help an ex junkie, he repaid her by knifing her and her son to death)
However a closer examination of your O.P should " Note" this comment " Why do religious people like shunning?"
A) Many in the organisation are incapable of shunning a friend.
B) I recently experienced shunning from a member of my former congregation. What a message that sent out. It was a very sad statement on the individual, because they know me and I know them, and the shunning was not balanced on rational thinking.