The images are unambiguously in the public domain? How so? The museum seems to think otherwise- and I (as a photographer and somewhat knowledgeable in copyright law for photography) tend to agree.
The moment a photographer presses the shutter button to capture an image the photographer owns the right to the image. The photographer- not the corporation who hired them. In some countries a blind photographer can tells someone else to press the shutter button and the blind photographer owns the copyright.
Now the photographer can assign rights, as per a contract, to the entity that has hired them to do the photography work- and that assignment can be irrevocable, single use, multi use, first press, etc. But no matter what when that button went down a copywritten work was created.
So what we have here is a very confusing summary of a legal letter claiming that the museum owns the copyrights and had the original, full size images taken by the photographer, available online but not directly linked. Their excuse is abhorrent, IMHO, to claim that knowing how to use a URL and download something is illegal. I don't think they have a leg there- but not knowing the particulars about the contract signed, who funded it (I'm assuming it was public dollars, but that's an assumption), the business relationship between the photographer and the museum... I think it's a very big stretch to claim their assertions are without merit.
A photographer lighting artwork may (and this comes from experience) spend hours trying to get all the nuances of the painting recorded properly. What would you say if the photographer had to take 9 consecutive images at different exposures and merge them all into a HDR-type image, then spend hours rendering it down to sRGB to view correctly on the screen. Brush strokes can reflect light- perhaps he had to cross-polarize shots carefully.
What I'm saying is that a photo of a painting is still considered a copyrightable item- you may wish it to be derivative to the 'public domain' but if that were the case any photograph in front of a public domain piece of work would automatically be public domain- and it is clearly not.
We don't know all the story, but it is very evident to me that he crossed the line. Intentions are good- I admire it- but definitely did something that was not in the spirit of wiki and may be against the law.
And no, whomever marked my other comments troll- this is not a troll. Just because I'm taking a stand against what you think "Free is right all the time" doesn't make me a troll. I'm providing thoughtfully logically laid out information for additional discussion.