Why Did Time Machine Delete My Old Backups?
After a full restore, a full backup is preformed (always). It's by design. This is also documented on the wonderful Podini - About Time Machine site, section TM is doing a full backup for no good reason. Other circumstances, that may trigger full backup, are listed too.

I quote:


  And Time Machine may do a full backup after any of the following:
   * A full restore, whether from Time Machine or a "clone" (probably)