Why Do Some Natural History Museums Prohibit Photography?

On Thursday I went the Anchorage Museum to visit the Mammoths and Mastodons exhibit currently on display there, and was told no photography was permitted.  I was told the Field Museum of Natural History was responsible for the policy, but some twitter communication with the Field Museum's media department has confused this point, so I need to get to the bottom of the issue of who set the policy first before I post more on this.

In general, I dislike no-photography policies. There seems to be no legitimate need for them, when one of the purposes of a museum is public education. More as events develop.