In some places, for some objects, it is a major issue of copyright and security. So something privately owned may be shown by a gallery or museum, and part of the agreement is that the owner retains all rights to any images etc, and/or no images can be taken of the cabinet/alarm wires/wall fixtures. (the security/images issue can also sometimes arise in insuring certain items) So the staff then have to try and prevent visitors from taking images of the object and it's surroundings.
Frustrating, but at least it gets objects from private collections, or which would otherwise be prohibitive to insure and publicly display, on show.