I just got back from NYC and went to places like NBC, where you couldn’t take photos. What is the REAL reason you can’t take photos? I understand in art museams where the flash MAY errode the old paint. But is there a real reason (example copywrite laws) or do they just want to sell you the pictures in the gift shop.
They want to sell you their postcards is why.
There could be a copyright issue if they don’t own the rights to some of the items on display, but I think you nailed it with your gift shop proposal.
It is said that the intense light exposure created by the camera flash is not good for old paintings and that it speeds up the decay process.
Is this true or not? Who knows, but that’s one explanation I heard.
Also, I know the rule is in place out of consideration for other museum patrons. With flash bulbs lighting up around you, it would be difficult to enjoy the artwork when your eyeballs are constanty adjusting and re-adjusting from the paparazzi-like atmosphere.
I believe the flash/fade thing. I used to have a LR carpet that got very noticably sun bleached because it was right in front of the big picture window.Now i dont know if photo flash=3 or 4 constant hours of daily sun exposure or not but it seems to make sence.Look what sun can do to your hair in just a few months in the summer? So it makes some sence i guess.
Well then couldn’t they just tell patrons not to use the flash?
All the ones I’ve been to do just that and if you use the flash anyway they’ll ask you to leave. I guess they’ve had too many people not listen so they don’t even bother trying any more.
Possibly also because many point-and-shoot cameras these days turn the flash on by default, especially if the camera senses it’s a low-light situation (like most museums). Say 90% of patrons remember to go and manually turn the camera’s flash off…well, that other 10% is still going to add up.
I’ve never heard anyone state this, but I suspect at least part of the reason is security. Try taking photos inside a bank. They’ll think you are casing the joint.
When you can’t take photos, take videos 
I just returned from Europe, and in general the museums (eg British Museum, Louvre) allow photography, even with flash. It’s the churches that generally don’t, especially the larger cathedrals. And don’t even think of shooting the ceiling of the Sistine Chapel (I did surreptitiously get a shot of the floor).