Many museums, particularly in Europe, ban cameras when the real objective it to protect paintings from flash exposure, from all those P&S cameras that make it difficult to disable auto-flash. (But they do not ban camera-phones!)
Protecting paintings may be one of their concerns, but excessive light exposure is only one of several reasons for such bans. There are also the annoyance factor for other patrons and the security issues.
I am a regular photographer in the UK's National Archives at Kew near London. There is no camera ban there but there is a ban on flash and tripods (people tend to fall over them and threaten to sue). Until recently you had to apply for a special photography sticker to be added to your reader's ticket. When I applied for this some time ago I had to sit through a boring lecture on why there was a flash ban. I pointed out that the main camera I use for document copying is a small sensor one with a built in flash. I suggested that before issuing me with the aproved photographer sticker that they should test that I knew how to make an exposure without flash. The sheer technicality of this suggestion frightened them and they admitted that most archives and museums prefer to operate an outright ban on photographic devices simply because their staff do not understand them.
Many such places have a long way to go before logic prevails... :-(
