Long ago they used to announce what you were disfellowshipped for.
If it was smoking - they said so.
Then when that became legally hazardous they changed it to
"So and so is disfellowshipped for conduct unbecoming a Christian"
when that got challenged they changed it to where it is today.
I think that because you "joined" when you got baptized, they are on terra firma legally to announce you are no longer part of the "group".
At least that is the way it was explained to me.