Why are there so many different views on Visitors Photographing..

Why do galleries take such different views on allowing or not allowing visitors to photograph:

The Louvre allows photography with common sense restrictions on the use of flash and tripod:

Still and video photography is permitted [in the Louvre] for private, noncommercial use only in the galleries housing the permanent collection.The use of flash or other means of artificial lighting is prohibited. Photography and filming are not permitted in the temporary exhibition galleries.

as does the British Museum

 [P]hotographs in the [British] Museum: Hand-held cameras, including cameras fitted with flash bulbs or electronic flash units, may be used for private purposes in the galleries unless otherwise indicated. Private photography in the galleries requiring the use of a stand, special lighting or other equipment is only permitted by prior arrangement with the Broadcast Unit.

 and the V&A.

 Photography in the [V&A] Galleries :You may take photographs or use a video camera in the galleries, but not with a tripod, monopod or supplementary video lighting.

 While the National Gallery in London doesn’t allow it...

 Filming and photography are not allowed in the [National] Gallery unless prior permission is given. To arrange a location shoot, please contact......

 Similarly   Tate’s Galleries :

 Photography and video filming are not normally allowed at Tate. Permission may be granted for the purpose of private study and research if no reproduction is available and copyright law permits.

Why the differences they are all public galleries, showing worked owned by the public ?