Willamette Week has a story up about how some people are bothered by the fact that the artist who created the Portlandia sculpture retains the copyright to it. It starts off sort of suggesting that this is something out of the ordinary, though later in the story it concedes that "actually, it's not that uncommon" (I would say it's not uncommon period). Cities can negotiate for the copyright to public art pieces if they want, but then that's going to severely limit the pool of artists interested in working with them.
UPDATE: Michael Rushton argues that it makes more sense for the city to acquire the copyright upfront. He says that "greatly reduces the transaction and other costs of anyone wishing to create an image that includes the statue having to negotiate individually with the artist." I'm not sure I agree. First, it only reduces the transaction costs if the city agrees to put the copyrights in the public domain (which I've never seen happen) -- otherwise instead of having to negotiate individually with the artist, anyone wishing to use the work will have to negotiate individually with the city. Six of one, half dozen of the other. In addition, suppose it would cost the city $100 to get the work without the copyright, and $150 if the copyright is included. Why should "we all" (that is, the general citizenry of the city) pay that extra $50, rather than letting people who want to sell posters and other tchotchkes depicting the work pay for it directly? Last, as I mentioned in my initial post, as a general matter, the best artists won't even consider such an arrangement. Rushton says "if the artist under consideration won't take such a deal, find another artist who will" -- but I think he understates the severity of that aspect of the problem.