Rights to use images of these objects, and even rights to access these objects are not always clear, and when clear, frequently prohibit access altogether. The monies received from license fees in one way or another are used to protect and care for objects. Monies received from issuing rights to publish very popular works one hopes end up subsidizing the costs of keeping and protecting less popular works. Even so, publication income does not pay the entire costs of warehousing objects. Think of it as an investment in future resources.
Museums also have other ways of controlling the use of their holdings.
One of these is to require, as a condition of entering the museum, that either no images be taken at all or that those that are taken be used in certain designated limited ways. For example, you might be allowed to use your images to teach a course, but not to publish a scholarly paper. The dividing line is a thin one, but sometimes, at some point, a repository's obligation to encourage the use of high quality images of objects in their collection manifests itself as a compulsion to limit the use of museum images for the benefit of learning and scholarship. A third manner of controlling the use of image content comes from the scholarly community itself. If I have read this post accurately, it is in this area that the author finds the most difficulty.