The reason for storing the css and images in the assets folder is because if you created your site based on a custom template (not the default ones), then we also need to store any css or images referenced by the custom templates.
However, you should be able to move the files and folders in DW, and DW will automatically update the paths for your. I did a test by removing the CSS and Images folder from the root to Under the Admin folder, and then moved the css and images folder from under the assets folder to the root and DW was able to update the paths correctly.