The standard explanation for the policies has been that the accumulated exposure of the art to the light would result in fading/decomposition of the paint/pigments/...and as you may have expected, there was some research to support the conclusions. But like so many accelerated exposure tests, this one was flawed, although I've not ever seen a study flawed in quite this manner:
"The flashguns were fired at the pigments every 7 seconds, and the total light quantity measured. At the end of a trial lasting many months it was found that these flashes had produced some change in the pigments. After more than 1 million of these test flashes, totalling 200,000 lux hours of exposure, about half the pigments showed changes just visible to the eye. The remaining pigments showed changes too small to detect with the eye, but measurable by photometry. In the vast majority of pigments there was no more change from UV-filtered xenon flash than from the same quantity of gallery lighting." (Emphasis added)That's right, the flashes did nothing to degrade the art, and yet this research led to flash pictures being banned. Pretty scientific, huh?
Martin Evans has some further thoughts to sum up the logic:
"Is it worth getting steamed up about such a tiny extra quantity of light, as far as pigment fading is concerned? Several photographers have already suggested that any trifling damage done by a few hundred of these little flashes in a day could be fully offset by closing the gallery and turning off the lights a few minutes early."But does anyone seriously expect this to change any museum policies?
[*] Last week? If you chase the references back far enough (Amusing News Aliquots --> Gizmodo --> Imaging-Resource --> Martin Evans, Ph. D.) , you see that some of this work goes back a few years.