Do I have this straight?
Publishers are allowed to make guidebooks and sell postcards without permission and also without recompense for the artists.
Historians and critics are allowed to include the photos in books without permission.
For-profit corporations are allowed to use the photos as long as they don't collect them into a single database marked "highly dangerous public database of art photographs".
But nonprofits who create a digital guidebook are copyright criminals - with the implication that they're not allowed to distribute the work for free, because it might affect the profitability of a hypothetical not-yet-existent commercial alternative.
I'd hope a good lawyer could sail a battleship through that line of argument. It's clearly prejudicial, discriminatory, inconsistent, arbitrary, and not in the interests of either the public or the copyright holders.
I suspect if it gets taken to the EU Courts, it will be shot down in flames in short order.
E.g. it might be ok to take a picture of each piece of art with a person standing in front of it, so retake the pictures or just superimpose a picture of a person with a transparent background onto each picture.
Then once you've found that loophole, exploit the heck of it and sell prints on the street with that person superimposed on the art. Just give a big middle finger to the law.
However I'm interested in what the case is with Google Ingress and their effort to make public art etc. more accessible through a smartphone-game using GPS and images.
Furthermore, yes, this is all madness.