Their goal is to change politically hostile environments by deploying in friendly cities. Then, residents of the hostile cities will start asking, "Why are we paying extortionate rates for shitty service?" Even if Google Fiber never gets to deploy there, the threat of it will hopefully be enough to get cable companies to act more fairly.
Well actually no they don't. They're going to prove their model is replicable and profitable, while not deploying to politically hostile environments.