Its not by design:
Public Folder granular restore data will NOT show up in restore selections if the backup job is written to tape. This is a limitation of MAPI. We are not able to get enough information from Active Directory to associate the Public Folder data with its actual location. This is not a problem with restore selections from backup to disk folders due to the dynamic way we can read the Exchange database using the VFF technology.
https://www-secure.symantec.com/connect/articles/understanding-grt-backup-and-restore-functionality-symantec-backup-exec
If you are restoring from GRT-tape backups, you need to run a duplicate job from tape to disk and restore public folders from the disk ..