btrbk prune --preserve-backups tries to read backup targets then fails

I'm trying to remove unneeded snapshots without my backup drive being connected.

```
% sudo btrbk prune --preserve-backups
WARNING: Skipping target "/media/backup/btrbk/rootfs": Failed to fetch subvolume detail
WARNING: Skipping target "/media/backup/btrbk/home": Failed to fetch subvolume detail
WARNING: Skipping target "/media/backup/btrbk/home-var": Failed to fetch subvolume detail
WARNING: Skipping cleanup of snapshots for subvolume "/mnt/btrfs-vol/rootfs/@", as at least one target aborted earlier
WARNING: Skipping cleanup of snapshots for subvolume "/mnt/btrfs-vol/home/@home", as at least one target aborted earlier
WARNING: Skipping cleanup of snapshots for subvolume "/mnt/btrfs-vol/home/@home@var", as at least one target aborted earlier
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.26.1)

Date: Thu Aug 23 16:08:40 2018
Config: /etc/btrbk/btrbk.conf

Options:
btrbk resume: No snapshots created
btrbk snapshot: No backups created
preserve-backups: Preserved all backups

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/mnt/btrfs-vol/rootfs/@
!!! Target "/media/backup/btrbk/rootfs" aborted: Failed to fetch subvolume detail

/mnt/btrfs-vol/home/@home
!!! Target "/media/backup/btrbk/home" aborted: Failed to fetch subvolume detail

/mnt/btrfs-vol/home/@home@var
!!! Target "/media/backup/btrbk/home-var" aborted: Failed to fetch subvolume detail

NOTE: Some errors occurred, which may result in missing backups!
Please check warning and error messages above.
% btrbk --version
btrbk command line client, version 0.26.1

%
```

Why is this looking at the backup targets given I've told it to `--preserve-backups`?

Am I invoking `btrbk` the wrong way?