Many things probably assume that debconf can be used by non-root users.
They're all making a dangerously wrong assumption.

In fact, debconf can be used by non-root users, in the default
configuration. This is because DbDriver::File goes out of its way (now)
to run in an implicit readonly mode if the database file cannot be
written to. An attempt to change something in the database will fail at
the end, but if no changes are made, everything works.

But, change /etc/debconf.conf to use a Directory or a DirTree, or
possibly other DbDrivers, and this assumption breaks down. Packages that
run debconf confmodules as non-root (or as fakeroot) will FTBFS. Other
stuff may break.

It's also quite possible that switching to cdebconf would raise similar
problems.

(See #577632, and #577299.)