Unless I am mistaken, this is by design David. If your users had or have the permissions to restore archived items, they are restored to the original location (host and container / folder). If this folder was deleted, the restores would fail.
You may have noticed that optional switches exist that permit you to delete, hide, or mark read-only - the PST file after it has been migrated but there are no parameters that permit you to delete the hosting folder.
Again I believe that this is by design. There are references within SQL that identify where the data item came from before it was archived so that it can be restored to the correct location. This would explain why you can't delete the folder or access it unless items are restored there first.
Anyone have any other comments or suggestions?