Many make the honest mistake of believing that because an artwork is outside it is public domain . It's NOT, in fact artists retain copyright no matter where their artwork is displayed, outside or inside, makes no difference. From a local viewpoint I can't tell you how upsetting it is for artists to walk into a local shop and images of their art being used for profit by others. Not only are they being deprived of income opportunities, any licencing fees, they aren't getting any promotional opportunies because they are not credited or their website listed. Does that mean people can't photograph public art at all? No, it means it can't be the focal point and used for commercial purposes (see example of mural below). This includes the mural in our local post office which the UPS has a statement about: New Deal Art: Murals and Sculptures.
After working many months rehabbing the old Holly's cleaners building for "would you, could you , IN A FRAME Les Gilford commissioned artist Jennifer Veutling and her sister to paint a mural of him and his crew on the side of the building. It was in celebration of completing a job they felt great pride in as well as stating who did the work. Scooby, Junior (Chris), Teaspoon(now deceased) and Les Gilford.
Some useful LINKS
Mural Copyright Laws
Legal Pitfalls in Taking or Using Photographs of Copyright Material, Trademarks and People
Visual Artists Rights Act "While there is principled justification for artists not being flexible about copyright ownership, practically speaking, copyright ownership in a broader context really only makes a difference within the public art context when there are merchandising issues at stake."
"A Simple Guide for Photographers, Artists, Illustrators, Writers, Musicians and
Other Creative Individuals " from Photo Law