How do you know what the government "uses"?? Hell, if YOU know about Linux/Unix, do you honestly think DHS does not?
Really, you haven't looked into anything at all. Microsucks has a contract with the government, it's the reason they have managed to stay afloat in spite of people switching to Linux/Unix or Mac by the hundreds each year (possibly thousands now, my stats on that are dated). The contract I think should expired in about 5 more years ... unless they renew it (if they haven't already). All federally owned computers are running Windoze, even their servers. The only reason this contract happened was because of two facts, at the time the government went to sign one Microsucks was the leader in OS development and the computers were all IBMs, and Bill Gates is a damned good salesman and (ex)CEO. Windoze started to fall apart much later (quite recently actually) because Bill stopped caring about Microsucks, which is why he turned control of the company to the idiot that has been making the really bad decisions lately.
That's how I know what their servers are running. I knew one of the people who installed the first Windoze into our local federal computers. He's dead now sadly, probably rolling over in his grave at how fall Microsucks has fallen.