Why all the fuss, you might be wondering? Because all the behind-the-scenes information about the documents you're reading is, some would argue, terribly private. It could easily be mined for ways to present you as a reader with more targeted advertising—and that information, associated with potential commercial gain, is a saleable commodity in its own right. The FTC again explains this neatly: "We have concerns about Google gaining access to vast amounts of consumer data regarding the books consumers search for, purchase, and read."
That's quite a change of heart by the search engine giant, and it could be taken to imply two things. First, that Google's becoming more of a gentle giant, keen to act to protect the privacy of its users. Second, that it's really trying to get the legalities right for a service it thinks will become big-business. My money's more on the second, probably because that's where Google thinks the money is.