Insight on the Scriptures Volume 1,p.787ExpellingOne who was cast out as wicked, cut off entirely, would be considered worthy of death, though the Jews might not have the authority to execute such a one. Nevertheless, the form of cutting off they did employ was a very powerful weapon in the Jewish community. Jesus foretold that his followers would be expelled from the synagogues. (Joh 16:2) Fear of being expelled, or "unchurched," kept some of the Jews, even the rulers, from confessing Jesus. (Joh 9:22, ftn; 12:42) An example of such action by the synagogue was the case of the healed blind man who spoke favorably of Jesus.—Joh 9:34.
They wrote the book. They know it is a "very powerful weapon" used to silence people from voicing their earnest convictions. Shammatta is the kind of expelling that most closely corresponds to disfellowshipping; disfellowshipping is only used when the GB would order death, if they legally could.