https://wol.jw.org/en/wol/d/r1/lp-e/1102014944?q=no+longer+witnesses&p=par
It appears to be for the same reason they mislabel people as "mentally diseased". If they presumptuously judge someone as "not a witness of Jehovah" then they can also say "not a brother or not a sister" and then they attempt to rationalize treating the person as sub-human, or dead. It's the same thing the Pharisees did when going beyond what was written about how to deal with leprosy or people of a different background or women.