Hi,
Copyrights are strange. My best guess at explaining them is that the founders of the US wanted items to become public domain.
The current copyright is until the author's (or photographer's) death plus a few years. I think it was death plus 25 years. For companies the time frame is about 25 years.
In about 2006 when I was scanning old Official Guides of the Railways to CD/DVD, the time was 76 years.
In other words, in 2015, anything published in 1935 and earlier is considered public domain. The display format (I think) can then be copyrighted but not the photos or text itself. Some of the earliest of Edgar Rice Burroughs mars science fiction (i.e. Princess of Mars) is now public domain.
The 1880 era photos of Jackson are now public domain. If you do not have the negatives, then copies are not as good. That is why SMU Baldwin items are still such a demand (for those items that are public domain).
Things might have changes and I might be confusing patents and copyrights with the details but the basics are correct.
Can a Government entity hold a copyright? Yes it can. The USGS maps, design factors of the SR-71 spy plane, and many other items are "copyrighted" by the government. This is (I think) where creative definitions occur. Keep the design specs of the SR-71 secret even though the plane has been retired is one way. In general though, I think the government has a 25 year copyright on items such as published census data.
Food for thought.
Doug vV