i'll throw my tuppence worth in here. i work in a museum and the photography rule changes depending on what the subject is. for instance if we stage an exhibition of picasso paintings from other galleries or museums then no photography is allowed because the lenders have not given permission.
however if we stage an exhibition of picasso items that we own, then you can snap away till yer hearts content..

would any painter staging an exhibition agree to everyone photographing their paintings? i doubt it.
so don't sue or shoot the messenger.it's the individual artist or owner of the pieces that decide if it can be photographed in most cases, not the institution.