You're right, it isn't. And, of course, nothing is stopping a pacifist from joining the military. After all, he can get a desk job. What I want to know is why.
Certain personality types are attracted to specific ideologies and in turn, specific careers. Why is somebody who is fascinated with wealth redistribution and income equality spend his time on a site which is devoted to the exact opposite on a personal level?
Simply drawing a dividing line between society and the self is not an answer. Most people have a congruent perspective when it comes social policy and their personal policy.
Why are these people different?