Per 17 U.S.C. § 105, any “work of the United States Government” is in the public domain from the moment it’s created. And yet, Google Book Search restricts access to snippet view: short one-inch sections around your search term. Compare the full download available for other clearly public-domain books.
My best guess is that Google hasn’t yet figured out that these government reports actually are public-domain. Here’s a hint for a future update to their copyright database: if it’s published by the U.S. Government Printing Office, it’s probably not in copyright. Open-source America’s operating system!