At one of those week-long (perhaps it was two weeks) school for elders I asked a similar question. "Why is it that when a JW John Doe joins the military that he is not disfellowshipped."
The instructor proceeded to explain that, instead, it is announced that John Doe has disassociated himself from the congregation. The two terms are actually synonymous when it comes to how the congregation must shun them. The difference is how the congregation is informed and it is purely legalistic. He proceeded, "can you imagine how quickly Watchtower headquarters would be shut down if they kicked someone out (John Doe is disfellowshipped) for joining the military? We're talking charges of sedition -- conduct or speech inciting people to rebel against the authority of a state. On the other hand, we can announce that John Doe disassociated himself from the congregation. We at headquarters simply listened to John Doe's apparent decision. That's totally on him. We're not responsible, simply informing the congregation about John Doe's voluntary actions."
The year was 1976 and I felt dejected. I felt like our leaders were putting their tails between their legs. Cowards, I felt. I left the org in 1977.