This is by design. Asus has simplified the interface so that you can only share and assign permissions to directories in the root, not subdirectories or root itself.
The only way through the webUI to share the root directory is to turn on "Allow guest login" (which shares the entire disk to everyone).
As you're using Merlin's firmware you could create your own shares by using a custom config file or script. But it wouldn't be reflected in the webUI.