There is practically no such thing as a truly private network anymore. The vast majority of networks, even government networks, have an internet connection somewhere. That connection may be under multiple layers of security, but its still a vulnerability.
It's true that there are secret and top secret level communications links available for transfer between networks of that level. However, they don't work the same way as normal internet connections (they aren't always-on, for instance), and they are completely segregated from any networks or communications links of a lower classification level.
At my work, moving information from the unclass network to one of the classified networks requires
1) Burning the data to a special CD bearing a media code and noting where that CD is going.
2) Finalizing the CD so nothing else can be burned.
3) Running a virus definitions update.
4) Scanning the CD for viruses after the update (no matter how little data you put on it).
5) Physically carrying the CD to a classified system and copying over the files.
6) Dropping the CD in a "bit bucket" in the secure area (it doesn't leave the area once it's been in a secure system, even read-only).
You don't even want to know how hard it is to arrange for any file (even if it was unclass originally) to be copied from the classified network to the unclass one. It's so hard that it usually isn't worth the trouble----far easier to print out the diffs and take them out on paper.
