Earlier quoted context omitted.
It's not low bandwidth link; it's intentional design. Secure computers are not able to connect to the outside world.
From the article: "...classified computers are often disconnected from the network, or are in low-bandwidth areas ."
Classified computers are almost always disconnected from the internet. In fact I haven't seen a computer with the combination of internet connection and approval to access classified documents.
So yes, they are low bandwidth. But all of the classified computers are networked together. Recall Pfc Manning pulled the files from a database that he leaked.
The bandwidth between the classified network and the nonclassified network is almost always zero or extremely limited (or might have to go through a person who literally weeds through the files one at a time and could accurately be described as "low bandwidth").
So any time you are circumventing that protection using removable media you are breaking the protocol. Just look at Stuxnet for the reason why other that Wikileaks.