The idea of copyright on a photo by its photographer is something the public - especially on-line - just doesn't understand.
The confusion is natural: a photo, at least a documentary life-as-it-is sort of photograph... well, it just doesn't seem created to an ordinary person. People can, generally, understand that a drawing requires an artist or that a clay bowl has a potter, but a photo...? Isn't that just life (almost by definition Public Domain) caught as anyone can with their camera-phone? (Anyone but me, I'm a terrible photographer!) Most people do not look at an unposed, unmanipulated, documentary photo and even think Photographer. They see only the subject. Which might well be in the public domain.
Actually, I kinda lean this way myself.
I tend to agree with Wikimedia and others' theory that a straightforward documentary photo that reproduces a public domain subject should, itself, be public domain. I mean, the Mona Lisa is loooong out of copyright; why should an ordinary just-the-facts-ma'am photo of her be copyrighted? The next 100 pro or amateur photos will be identical to it. And how else can the Public of the public domain interact with Mona without some photo? Not like the Louvre's going to let anyone take her home on loan.
Or like paying to use that generic photo will help Da Vinci pay rent. The point of copyright is to help support the people - artist or engineer etc. - who make a New Thing. Copyright is intended to encourage innovation.
Now Andy Warhol's version of Mona Lisa? THAT obviously deserves copyright because it is unique to Warhol and his experience of the painting. Likewise, if a photographer took a non-documentary version of Da Vinci's painting... strangely and uniquely lit, perhaps, or manipulated in the lab or digitally into some personal vision, then that too would be copyrighted. (Please remember these are my personal musings here, not actual copyright Law, which is weird and sometimes deeply stupid.)
Likewise, to my way of thinking, most what I'll call Travel Photography ought, I think, to be uncopyrightable. The pyramids, for instance... It'd take an awfully special photo to see those in a unique way after humans have been staring at them for millenia and photographing them for more than a century. Still possible though.
Photographers are in a similar position now to where portrait painters were when photography was invented: the mere recording of fact is suddenly possible for anyone and therefore devalued; what IS still valuable is a unique artistic viewpoint. Sargeant's painted portraits do not just record facts about his sitters. Ansel Adams' photos of Yosemite do not just record the shapes of rocks.
I dare all the camera-phone folks to match their insights.
Meanwhile, here's a reminder of actual copyright law online or off - as I understand it. (I do try to obey copyright law.) All those pretty pictures out there, a click away, were taken by someone. And those someones have the only right to use the image unless you get permission or they have given it a Creative Commons license (read that fine print) or declared it Public Domain OR (and this is a big, messy, legal-and-artistic OR) you use it in a Fair Use way.
Fair Use is a whole 'nother post!
Anyway, in the process of my emailing on this topic, I created a little sketch to replace a photo so that I would have no copyright questions hanging. Here's that sketch:
Sketch of alphabet blocks by Clare Floyd DeVries - gifted to the Public Domain
In the spirit of... neighborliness or something, I want to just hand this sketch out to anyone - do what you want with it. As much as the law allows I gift it to the Public Domain. Fly! Be free. If you want to credit me, that's kind; if you find a way to make a gadjillion dollars off it and want to send me some bucks, thank you very much, but you don't need to. It's all yours.
ADDENDUM x 2: For a fascinating legal discussion of copyright, fair use, and photography read this article at Art in America HERE about the recent court decision in the Richard Prince case, which finds in favor of "transforming" photos by others into new works. And HERE is another court decision that is chilling to claims of photographers for the "transformative" nature of photography itself. In this case the photo was of copyrighted art by someone else. (My gut feeling about documentary versus unique view-point manipulated photos may actually hold some legal water.)
Anyway, photographers and photography are clearly under siege today... And the front line may be the internet.