Well that makes them citizens of their own communities. Certainly Megaton sees itself as a community and Simms is the Sheriff and Mayor, or so he says, and the citizens of the town accept that. Canterbury Commons is organized as well although they all roll their eyes at Uncle Roe behind his back. Andale is very organized in it's weird little way. Rivet City has a City Council. Big Town spends most of its time worried about building up its defenses. It doesn't have much in the way of civic organizations but the kids there are organized and recognize themselves as a town. Little Lamplight elects its mayor. The CW doesn't need the Enclave, but the Enclave really wants the CW. It's the way the story in the game is written. It could easily have been written so that the LW helps the little communities scattered around the Wasteland come together to help one another in some way. Maybe via the trader's caravans. But that wouldn't have been as exciting as Vertibirds and Power Armor.
Truthfully i'd a preferred that. Having to fight off Supermutant hordes from destroying the very semblance National Government you have created.
But you misread my intention. Without a National government they cannot call themselves Citizens of the United States. Yes, they are Native to the states, and their ancestors may have indeed been Citizens, but now they are like you said, Citizens of Megaton, Rivet City, Big town, Andale, ect...