Even if it is, the government would have easier access to and better control over an open-source stack than it would with Windows. It would also have a wider possible market because support vendors would not have to get Microsoft's blessing before providing services.
Actually, the US government probably could get Microsoft to make a change / add something a lot quicker than it could from the Linux community (money is a fine leverage). Many support organizations don't exactly get permission from Microsoft to bid.
Red Hat/Novell/Canonical could add/change Linux features just as easily as Microsoft could add to or change Windows. The Linux community isn't just a bunch of people who are writing code for the fun of it.
Not only that, but you can make Red Hat, Novell and Canonical compete against each other for the best solution.
As much schizophrenic as Microsoft's various divisions are, you can't make the Xbox division compete with the Office division on which one wins a contract for adding a feature to the Windows 8 kernel.
For better or worse, the US government has a lot easier time dealing with a central point of control and owner's of products are their institutional experience. Also, I gotta wonder how a change that was not popular would work if made by one of the distribution companies.
