I'd also be interested in this. Camera restrictions, or more positively camera guidance, are interesting in ecommerce for instance. It avoids users getting lost in a model. This tends to happen when a user can pan and orbit the model. It's possible the user pans an object completely out of view.
It's also very practical with the graphic design of the website the iframe will be placed in. Currently a user can pan the model in such a way it touches or overlaps the edges of the iframe. This behavior is not always easy to integrate in a good graphic design.
Finally, in some cases we just want to show the product from specific camera angles, but not others. Or we only want to show the upper hemisphere of a product. Disabling camera movement or limiting it to certain angles makes sense in those cases.