"Shunning" is a real policy advocated by some groups, such as JWs. It's intended to punish the traitor, and the rule is that you are supposed to completely divorce yourself from that person, cutting off all forms of contact. The intent is to cause the person to realize the error of their ways via tough love and come back.
Mormon policy is unequivocally NOT shunning. It could not be more opposite. The Church regularly implores its members to be tolerant and kind to everyone, including family members and friends who've left the Church or quit practicing.
It's true that Mormons are one of the only religious groups in the U.S. that, by and large, continually integrate it into their daily lives and take it seriously. That makes it a bigger deal when someone leaves than in mainstream American Christianity, where "church shopping" is very common, and there is a doctrine of a "priesthood of all believers", i.e., as long as you continue to claim to be Christian you're A-OK. Mormonism does insist that Mormonism is important, and most Mormons believe that.
But I think the parent is correct. This is to be expected whenever you challenge, insult, or disavow the deeply-held beliefs of any group. It's very difficult for everyone when those beliefs are held by close friends and family members.
That many Americans do not care if someone leaves their religion merely demonstrates that they consider religion a non-critical social function instead of a set of core beliefs and practices to guide one's life.