Soldiers and so are not professional IT men, so they can be told that MS is a big company, BAE is so professional, they can do something professional. Anyway, that's the truth maybe.
Linux is a no-go, as it's community written, and such is not good enough on paper, who will support it etc. Redhat cannot tell the military that 'no, we didn't make this but try to support' :) It's a nuclear submarine! MS did it, BAE did it, they can support it! Or not.... :)
I could sleep better with a solaris for example...