From a pragmatic standpoint, how do you separate the cases of:
1) Someone taking holliday snaps while in Manhattan, publishing them on Facebook (should clearly be legal)
2) Someone doing a photographic reproduction of a painting, selling copies on Ebay (should probably be illegal)
As I understand, it's quite straightforward that a copyright period should apply to all works of art (paintings, photographs, literature, music, sculpture etc), and after that copyright period has expired, which is usually 50 or 70 years after the death of the creator, one is free to sell copies of the work.
This is why we can have that wonderful site, Project Gutenberg, which makes available all the great works of literature throughout history, for free.
However, there seems to be a problem with regard to works of art which are still under copyright but are displayed in a public place, such as a painting adorning a wall, or a sculpture set in a park or a city square.
It seems, as I understand it, that to photograph and sell an exact copy of such painting or sculpture, when the painting or sculpture is clearly the main focus of interest rather than part of an interesting background, is illegal.
However, here is where things can get silly. Who determines what is the main focus of interest? If I crop off the surrounding background from a photo of a painting on a wall in a public place, so that only the painting is shown and nothing else, then it's quite obvious that the painting is the main focus of interest.
But suppose there happened to be a large butterfly that settled on a part of the painting, perhaps on the nose of a person depicted in the painting, then presumably I could sell my photo of the painting. But not necessarily.
Perhaps the painter who had the copyright could take me to court on the grounds that the presence of the butterfly was incidental, and that his painting was still the main focus of interest. We are then into the realm of pure opinion, existing only in the mind of the beholder. I could counter by claiming that I would never have bothered photographing the painting if the butterfly had not landed on it, and that the butterfly was much more beautiful than his painting...and so on.
Likewise, the same situation could apply to a sculpture in a botanical garden; even more so because a sculpture is a 3-dimensional structure and always has a visible background when presented within a rectangular frame, unless such background is rendered completely out of focus.
There could be a play of light and shadow creating an interesting pattern. The photographer who took the photo of the sculpture could claim that he would never have bothered taking the shot had it not been for the unusual lighting which made the shot interesting. The lighting was the main focus of interest and the sculpture was merely the background on which the lighting effects were displayed... and so on.