The Copyright explanation, has become like 'thats a Health and Safety issue' in orther situations. Its a blanket cover all excuse which may or may not have any merit or indeed accuracy. It is very difficult to imagine, for example, how there could be a copyright situation in photographing an armoured car or aircraft on display. They are also going to suppress an awful lot of goodwill for themselves among, say, school groups if they do not let pics, at least in the 20th century gallery.
