Why can't I assign a directory to the root directory?

The account you are trying to configure is probably set to operate in Simple Virtual Directory mode.

The virtual directory (VD) system allows the administrator to attach any directory or drive to the root. When a client requests the root directory from the server, the VDs you specify are sent to the client. The client can also navigate to any of the VD directories' subdirectories. The VD system takes care of all path translation.

Security settings can be specified for each virtual directory. All subdirectories under the VD inherit the security settings of the VD.

There are 2 modes that a user account can operate in with respect to the virtual filesystem. The two modes are simple and standard mode.

Simple Virtual Directory mode

When a user account uses simple directory mode, the administrator can only assign one directory to represent the virtual directory for that user. Instead of that directory being seen as a subdirectory off of the root, the virtual directory selected will be the directory the user is placed in when they first log into the server. In other words, the directory selected as the virtual root directory will be the root directory.

Standard Virtual Directory mode

In standard mode, the administrator may add as many directories as virtual directories to a user account as desired. The directories selected will appear as subdirectories off of the root when the designated user logs into the server.