‘no photography allowed’

So here’s a question… how come at some exhibits there is ‘no’ photography allowed? most museums I have ever attended (domestic and international), photography is allowed, just no flash. But how come some specific exhibits can not be photographed? Is it security? certain insured work that can’t be documented?

For example, Just went to the DALI exhibit at the MoMA 2 weeks ago, and I wanted to document his details for you to see, but no. no cameras allowed… I can understand no flash. but nothing at all? how is an artist suppose to document a piece for inspiration. sure, buy books, but if you are struggling? Take books out from the library? sure, then why are other exhibits allowed to be documented and others not. The only reason why I ask, is ’cause I haven’t seen much consistency with this rule… can anyone enlighten me?

ps: Salvador Dali exhibit currently at the MoMA in NYC is a great collection of his paintings, drawings and film. don’t miss it.