But it didn't require military service.

In fact, (according to the book) most people who wanted to serve were NOT pulled into the military but were send to work on (extremely dangerous) terraforming projects or with field trials of other things.

Oh, and they HAD to accept anyone who applied - they might not get into the military, but they'd find SOMETHING for you to do.

I suspect that Heinlein's point was that some people scorned citizenship because it had no value to them...they simply received it without ever having to "earn" it.