Those boxes indicate the size of the new image after cropping. 0 means the same size as the crop selection. using those boxes you can make the cropped image larger or smaller than the crop selection within limits from config settings. So the boxes do not have any effect on the crop selector itself, just on the resulting image size.

its just javascript selecting the crop points, it doesn't really know anything about the image, the cropped image is created on the server based on the selection points.

We can force an aspect ratio and we do for example where users can upload an avatar/photo if using the internal avatars, we force that to be square because we want all the avatar images to be the same size and shape.

But it makes no sense to put that kind on restriction as a policy on other images in content when we know nothing and can make no assumptions about what the user wants.