Freedom of religion is not absolute if it can be proved harmful to an indiviual and or society (which this can) than it can be banned so say the supreme court.
This is not the correct test that the Supreme Court uses. The test is whether the law that infringes upon religious practice is a neutral law of general applicability that is not specifically targeted at a particular religion. So to use your analogies, cannibalism and polygamy are illegal regardless of whether they are practiced for a religious reason or not, and the laws were not enacted to target religion - they are generally applicable. On the other hand, cutting off contact with a relative is not generally unlawful, and if a new law were enacted prohibiting shunning, it would arguably be targeted at religion in particular, and not general behavior in society. That's why shunning is legal under the current U.S. legal framework and will continue to be for the foreseeable future.