why is /dev umounted before switch_root in initfs?

I noticed a problem in an unreleased version of Portfolio that seems to be caused by the fact that info in /dev is "lost" after switch_root from the initfs init.sh script. Specifically, when using FDE, the rootfs luks vol is unlocked and a node is made under /dev/mapper, which is ultimately mounted as / (new root). /dev is then umounted before switch_root, so that node is no longer available in /dev when pmOS fully boots:

librem5:~$ ls /dev/mapper/
control
librem5:~$ df /
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/root      29874052  22703688   5637852  80% /

this seems to be screwing up udisks2, since it's unable to figure out what the backing encrypted volume is:

/org/freedesktop/UDisks2/block_devices/mmcblk0p2:
  org.freedesktop.UDisks2.Block:
    Configuration:              []
    CryptoBackingDevice:        '/'
    Device:                     /dev/mmcblk0p2

CrytoBackingDevice should point to the block device under /dev/mapper: http://storaged.org/doc/udisks2-api/2.9.4/gdbus-org.freedesktop.UDisks2.Block.html#gdbus-property-org-freedesktop-UDisks2-Block.CryptoBackingDevice

For example, on my laptop running Alpine:

/org/freedesktop/UDisks2/block_devices/dm_2d0:
  org.freedesktop.UDisks2.Block:
    Configuration:              []
    CryptoBackingDevice:        '/org/freedesktop/UDisks2/block_devices/nvme0n1p2'
    Device:                     /dev/dm-0

switch_root should move /dev (and other things like /proc, /sys) to the new root. the git history for the relevant section in the init.sh was lost(?) in 1c0ff6aa:

umount /dev

# shellcheck disable=
exec switch_root /sysroot "$init"

So, does anyone (@ollieparanoid ?) remember why we are umounting /dev prior to switch_root?