Seems that the US government is finally taking security seriously and choosing Macs. Regardless of the alternative (non-Windows) platform there is an inherent lack of security in a homogenous Windows environment.
The Army's push to use Macs to help protect its computing corps got its start in August 2005, when General Steve Boutelle, the Army's chief information officer, gave a speech calling for more diversity in the Army's computer vendors. He argued the approach would both increase competition among military contractors and strengthen its IT defenses.
I would still ask why the government wouldn't just create their own Linux distro, but Macs are a great start to a more secure environment.