Honestly, as a stock photographer, I'm all about protecting my copyright. However, I also recognize the "fair use" doctrine.
In this case, the photographer did not take a picture of the "footsteps art" embedded in the sidewalk—as a primary subject. His photo was of a model dancing on those footsteps, doing what the city that paid for the art intended it be used for. Only part of the artwork shows in the image, and it is clearly not the primary subject, although it is being used by the real primary subject.
This appears to be a case where an artist, Jack Mackie, is getting a little greedy and looking for an additional revenue stream. He is currently suing 30 people for similar "infringments" of his copyright. Mr. Mackie has copied the dance steps for the Waltz, Tango, and FoxTrot. Did he invent those dance steps? Nope! Did he invent the shape of a shoe that he embedded in the sidewalk? Nope! My point? He is using ideas created by others. Fair use? Yes.
As an author, I wouldn't be happy if someone photocopied one of my books and sold it. However, if a photographer happened to take a stock picture of someone reading my book, and then sold the picture, I'd not object at all. I'd consider it free advertising.
The photographer, Mike Hipple, has a donation button on his blog, and I donated $10 to him. Read the story and help the guy out. All of us have taken pictures with someone's "art" in them. "Fair use" must be protected as well as copyright, or we might as well put our cameras away for good.