This is by design, when restoring items from a Granular backup of Exchange that was done to tape Backup Exec has to stage the entire information store to a temp location. Once this is completed then it will restore the items selected for restore.
From the Admin's Guide Page 1264:
If you restore individual items from an Information Store backup set that is on a device other than a backup-to-disk folder, then Backup Exec must temporarily stage the entire database to a path on the media server to extract individual items. Because of the potentially large file sizes that are created in the staging location path, system volumes should not be used as a staging location.
