The article points out that only "20,000 of the Army's 700,000 or so desktops and servers are Apple-made". This likely means that they have 20,000 Macs at the Pentagon alone, where the security is needed. Those other hundreds of thousands of computers probably belong to recruiters or low-level contractors, whose data is not too critical to national security. The Army would have no intention to spend money to upgrade systems, such as those belonging to recruiters, that don't have very sensitive data on it. I foresee that this Mac craze will be short lived, although I am not doubting it's impact on security.