Well, defending it is something else entirely. Being sent to a remote outpost in Afghanistan without even a broad objective for any US soldiers being there to complete is only arguably "defending" and quite a poor argument at that. The country, itself, is well defended without me.
It seems deeply wrong to continue to participate and to pay taxes into a system that one has no confidence in. What seems more likely is that morality never played a role in this decision, and instead its a scapegoat for not taking personal responsibility for one's role in the system.
I have a moral responsibility not to participate in unjust wars, it absolutely was a moral decision. I assume there are very many like me.