maybe they just don't want the congregation to know if a person is out by choice or force.
Yes, this makes sense. So many are leaving of their own accord and disbelief and not because of some biblical sin commited. This is what the WT does not want its members to know. Now the congo will just speculate about everyone who is is annouced as "no longer one of JWs" and imagine them as being wicked and evil and unrepentent even if there was no "biblical sin" commited. It will help the WT image to demonize everyone who has left the Org for whatever reason and make sure the congo is kept in the dark kept clean.
There may also be some legal slander type issues they are scooting around.
cybs