You might think that, but in fact the hook shipped with btrfs-tools only copies in /bin/fsck.btrfs if /usr/share/initramfs-tools/hooks/fsck is not executable. It is executable, so this has no effect.
I have checked on current wily with / on btrfs that my initramfs really is missing fsck.btrfs (in either /bin or /sbin).
You might think that, but in fact the hook shipped with btrfs-tools only copies in /bin/fsck.btrfs if /usr/share/ initramfs- tools/hooks/ fsck is not executable. It is executable, so this has no effect.
I have checked on current wily with / on btrfs that my initramfs really is missing fsck.btrfs (in either /bin or /sbin).