Hmmm. I know what I'll do.
I'll take a 4x5 view camera, set it up on a tripod, and take perfect copies (or as close to that as is possible) of photographs by masters of black and white photography like Ansel Adams, Edward Weston, and Robert Mapplethorpe. Then I will make prints of these photographs, so that my prints are indistinguishable from the originals. I'll sign them with my name and sell them as my work. Yep. No one will guess that they are not mine - I'll make a fortune - that's why museums don't allow photography.
</troll>
Seriously, museums have to make money. Admissions do not come close to covering expenses, nor will they ever. They need to create a source of revenue and what they have of value is the art.
The copyright on a 200 year old painting is long expired. But the copyright on a photograph of that painting is still legitimate. So the museum restricts what sort of photographs can be taken. Photographs taken by handheld cameras are allowed, as, with the lighting conditions in the museum, it is impossible to get publication quality photographs with a handheld camera. Photographs taken using tripods are disallowed, as they could be used for publication. By supplying the photographs used for publication, the museum can generate revenue.
Maybe you should be able to get perfect photographs of all the works of art in a museum - you can. For a mere 50 cents, usually, you can get nice postcards of the major works in a museum's collection. This is only slightly more that your snapshot costs you. Or perhaps the museums should not need the money... that would be nice... how about bringing the funding of the National Endowment for the Arts up to 1% of the national budget? | 
