I can assure you that the DoD cares about security. I am curious about why Windows would be "too expensive" to support.
Two reasons: 1. With system files, file locking, the registry, and similar complications, Windows makes backup software far more complicated than necessary -- in this case, "expensive" means "it would take a long time to write all the necessary code". 2. Windows users are generally less clueful, and therefore require more time-consuming hand-holding.
EDIT: There's also another benefit to starting with FreeBSD: Many FreeBSD users know and trust the project's security officer, while most Windows users haven't a clue who I am. :-)
And you earnestly believe that forcing them to use BSD would make them somehow better able to grok computing?
My customers aren't using Windows.
... ok, I don't have any customers yet. But when I launch, FreeBSD and Linux will be the only supported operating systems (with other BSDs to follow as soon as someone cares enough to ask for them; and OS X on the near-term to-do list). Windows is too expensive to support; and in any case, people who care about security probably aren't using Windows anyway.