Improved control over what and how the machines are setup.

Improved resource usage, allowing longer life for the consoles.

If separation of system vs user is retained, trojans and other things become a moot issue, as the users can just be removed and new ones created. (Not I didn't say a NON-ISSUE)

The recent outbreak of a credentials grabber trojan/virus... that kept showing up and was caused by a removable drive... which originally was though to minimize the Virus problem, in fact exacerbated it. If you plan for and design a system for a network from the beginning, you have a better chance you'll be able to minimize risk, versus bolting on networking and "internet capabilities" as an after thought.

Pretty much that sums up why. Though they probably don't release that info.