Is public domain software DFSG-compliant?

[Karl Fogel <kfogel@floss.red-bean.com>]

Is public domain software DFSG-compliant?

Sorry if this is a FAQ; I couldn't find the answer anywhere on the
Debian web site.  I am asking after reading this text on
http://www.debian.org/intro/free:

   "Truly free software is always free. Software that is placed in the
    public domain can be snapped up and put into non-free programs.
    Any improvements then made are lost to society. To stay free,
    software must be copyrighted and licensed."

The DFSG comprehend many licenses which don't meet this criterion.
Heck, most of the non-GPL licenses don't, I think :-).  So wouldn't
the dynamics of public domain software be the same as with any
DFSG-compliant but non-copyleft license (say, the MIT license)?

That is, anyone can take an MIT-licensed program and incorporate it
into a proprietary program.  They are not obligated to change the
license of their proprietary program, nor to show any of their source
code -- not even the portion that came from the MIT-licensed software.

Yet the original MIT-licensed program continues to exist, out there in
the world, and is as free for anyone to use as it ever was.  It's not
as if the moment a proprietary program incorporates a copy of the
MIT-licenced code, all other copies of the MIT code suddenly cease to
exist.

Thus, public domain code should be able to "stay free" in exactly the
same way MIT-licensed code would, and be considered DFSG-compliant in
the same way as many other non-GPL licenses are.

Or am I missing something important here?

Please CC me on any responses; I'm not subscribed to this list.

Thank you,
-Karl Fogel