Google has been buying so called dark fiber for years, when you run fiber the cost of adding extra strands is trivial compared to the digging etc.
Google probaly won't do more than the demo project for this however. What this does do it shows that google can do it. So when the the big ISPs make the attempt (like they have already) to make Google and other big sources pay for allowing the ISPs customers to reach them they can point out that they can bypass the ISP and hurt them badly.
