I don't consider a real-life comparison to a country which has nothing to do with the fictional Zoners as "overwhelming logical reasoning".
Zoners have lots of territory to defend - their guard systems and all the Freeports about. The idea of the "mysterious Juggernaut" hiding in the nearby nebula, waiting to strike out and defend the Freeport from any interlopers, actually makes sense when you think about it. Diplomacy can only get you so far - there are some people who will only listen to the sound of gunfire.
Individual Zoner capital ships may be strong, but the Houses have many more capital ships and the ability to produce them, so the Zoners aren't exactly comparable with the Houses (especially not on a localised scale).
The Zoner juggernaut is also used as a colony ship already. Read the infocard?