The "Privacy Policy" for this data is on the U.S. Census website here:
http://www.census.gov/about/policies/privacy/data_protection...
Note the statement that "the statistics we release do not identify individuals or businesses".
During the last census, it became pretty obvious that this would be an effective way for the U.S. Government to compel information from people who were suspected of something. Section 215 of the Patriot Act, after all, gives the government carte blanche to obtain any "tangible" thing.
IMHO... the interesting questions to ask when this legal opinion is released are:
1. Was there ever any accuracy to the U.S. Census "Privacy Policy"? Were the privacy policies of the U.S. Census misrepresented to the public, and in particular to those who were required to complete an extended survey?
2. Does the legal opinion address the "third-party doctrine" when the information provided is provided under legal compulsion, and in particular with potential (if commonly unused) penalties for non-cooperation?
A very interesting test case would be for someone to sue the government, in light of the upcoming 2020 census, to test the constitutionality of the criminal liability for not completing the extended survey in light of the government using the information provided for potential law enforcement purposes. It should be easier than usual to establish standing since anyone can be forced to complete one of these extended surveys.