Apple - Why is my Hard Drive and User folder shared when I turn on osx file sharing?

Prior to Mac OS X Lion, Mac OS X Client came with SAMBA Server and Client and what you defined to share was all that was shared. Since Mac OS X Lion, and continuing with OS X Mountain Lion and other releases of OS X, SAMBA has been replaced with SAMBX due to SAMBA moving to GPLv3. As a result SAMBA is basically broken in the OS since 10.7. The Mac OS X 10.7 Server and OS X Server going forwards does have some finer sharing controls not present in the Client version of the OS.

As a result of these changes the Macintosh HD and User's Home folders are automatically shared for them as an Administrative User and only the Home folder as a Standard User, in conjunction to what one chooses to share in System Preferences > Sharing > File Sharing.

To see the additional shares including what's defined in System Preferences > Sharing > File Sharing and what's not defined there. In a Terminal, the output below shows what's shared for me as an Administrative User and only sharing a folder named Temp in File Sharing. With File Sharing enabled in System Preferences > Sharing, just copy and paste the following command into Terminal and press Enter: smbutil view //$USER@$HOSTNAME

$ smbutil view //$USER@$HOSTNAME
Password for $HOSTNAME: 
Share                                           Type    Comments
-------------------------------
Temp                                            Disk    
IPC$                                            Pipe    
Macintosh HD                                    Disk    
$LOGNAME                                        Disk    

4 shares listed
$ 

If you do not want those shares exposed then do not turn on File Sharing via normal System Preferences and instead install SAMBA and manually configure file sharing yourself.