New York City's subways, of course, have long been contested subjects for amateur and professional photographers, with the MTA having unsuccessfully sought a photo ban in the decade following 9/11—though cops still regularly threaten people with arrest for taking pictures on the subway, I've seen it happen. The cops in Long Beach are evidently equally vigilant against the prospect of terrorist reconnaissance: Police Chief Jim McDonnell confirmed to the LB Post that official departmental policy is to stop photographers and videographers taking pictures "with no apparent esthetic value."
(Other suspicious activities, per the Long Beach PD protocols as reported by the Post, include using binoculars, "asking about an establisment's hours of operation," and "taking notes.")
Pressed for comment by the local paper, to which the detained photographer is a sometime contributor, Chief McDonnell clarified that there is no set definition for what is art and what is terrorism: officers just know it when they see it. The Post notes that "while there is no police training specific to determining whether a photographer’s subject has 'apparent esthetic value,' officers make such judgments 'based on their overall training and experience.'"

So then. Does the photo have "no apparent esthetic value"? Only if you hate Robert Motherwell's "Elegy to the Spanish Republic," I would submit. Clearly the responding officer is an acolyte of Manny Farber, who in his famous essay "White Elephant Art vs. Termite Art" lamented that:
The private voice of Motherwell (the exciting drama in the meeting places between ambivalent shapes, the aromatic sensuality that comes from laying down thin sheets of cold, artfully cliché-ish, hedonistic color) is inevitably ruined by having to spread these small pleasures into great contained works. Thrown back constantly on unrewarding endeavors (filling vast egglike shapes, organizing a ten foot rectangle with its empty corners suggesting Siberian steppes in the coldest time of year), Motherwell ends up with appalling amounts of plasterish grandeur, a composition so huge and questionably painted that the delicate, electric contours seem to be crushing the shalelike matter inside.
Wolff, in the end, was not detained, probably because that's an awful lot to fit on an arrest report.