Maybe each city is patrolling the their border? It is a civil war after all, there should be 6-10 men patrols going around the main roads and whatnot they're at war.
Maybe, but they make it out to be more like political infighting and coups, rather than a full-scale war that would require borderwatch duty.
Just the general feel of Skyrim, it wouldn't surprise me if the Nords are a lot more Darwinian in their views. If you can't survive out there, you're no use to us here. That sort of thing.
Or maybe I don't give the Nords enough credit for being a totaly civilized and not bloodthirsty race of hyperangry viking warriors.