Because Jehovah's Witnesses believe that one dedicates their lives to Jehovah and then symbolize that dedication in water baptism. Jehovah's Witnesses do believe that as a united people they too have dedicated the organization to Jehovah God and for the advancement of the work that he provided those that profess to be his witnesses to be. When one chooses to stop identifying themselves as a member of that organization formally, there has to be actions to protect those that continue to want to associate with that organization from any possible bad influence. Because Witnesses cannot read hearts and fully understand why people want to formally leave that organization they have made a policy that all individuals who take that step must be treated the same. That is why there is no announcement of why someone DA themselves or was DF, because that may not be the full reason why someone chose that or why the elders decided that the person was no longer repentant, the only important part is that the announcement is made that they are no longer one of Jehovah's Witnesses. These individuals may be very good and fine people, but no one can read or know the full story of why someone chose to make that decision of the formal removal of themselves from that organization. The reason stated in the formal letter may not be the real and true reason and may want to encourage others to leave or they may not, again it is something that no one can know for sure. And again it is a policy that anyone regardless of the reason, including if they are a victim of child abuse, is treated in the same way, again because no one can fully know why they chose to do so.
it is also the fact that Witnesses do not believe in interfaith activities. Witnesses who get baptized after they were part of another religion is encouraged, not sure if required, to formally remove themselves from the other religion. Witnesses believe that formal removal of a religion's rolls releases them from that religion's policies and guilt before God. So they have a formal way of removing people who don't want to be witnesses anymore.
Again it doesn't matter, what the answer is, you won't accept it as being a valid one.