One of the few bright spots in United States copyright law has always been that data produced by the government is, in theory, in the public domain. While there have of course been encroachments on this doctrine from time to time, it has generally been been held to in practice as well as in theory.
Unfortunately, being in the public domain isn't necessarily the same as being online and accessible in reasonable formats via modern protocols. For example, Carl Malamud has spent a fair amount of effort prying the raw records of copyright registrations out of the U.S. Copyright Office at the Library of Congress and putting them online in a much more useful way than the government ever had. Similar stories abound among those with experience extracting electronic data from governments.
The purpose of the Open Government Data Principles is to clearly and precisely articulate what the standard should be for governments to make public data available — to promote a standard that government agencies can live up to, and that constituents can expect. These principles do that, but they're just a start: now we have to actually meet them!