The article said they actively blocked others from even taking photos.
"The museum has long prohibited visitors from taking any kinds of photographs of its biggest attraction."
So they are asserting copyright of the work in the sense they are restricting access.
Their only real reasonable excuse might be that flash photography is likely to damage the work but non-flash photography should still be allowed. They've got history of blocking all such access.
I was in a museum that said photography is forbidden because of the fragility of the objects in a particular exhibit. I thought it was weird/amusing that it had no indication (even in an icon or something) of flash photography being the problem.
The article said they actively blocked others from even taking photos.
"The museum has long prohibited visitors from taking any kinds of photographs of its biggest attraction."
So they are asserting copyright of the work in the sense they are restricting access.
Their only real reasonable excuse might be that flash photography is likely to damage the work but non-flash photography should still be allowed. They've got history of blocking all such access.