Question: why were these computers connected to the Internet in the first place? So the drone design engineers could check their Facebooks? Computers doing real work should be airgapped, this is a no-brainer.
Another guess is that, because so many defense contractors and manufacturers are interdependent these days -- almost nobody designs or builds complete systems anymore -- they are tied to each other through communication channels and networks. A really good hacker-spy could piece together bits of information from Source A with information from Sources B, C, D, and discover or infer vulnerabilities from there.
Air-gapping does seem like an obvious choice, however, and I'm legitimately puzzled if it's not being done for the most sensitive design systems.
For example, let's say you run a quant trading firm and the algorithms you're concerned about being stolen need connectivity to download live trading info, and then after processing that info they need to communicate buy/sell orders to the outside world.
Are there any methods that could be used that would prevent all communication with a secure system (with an airgap level of certainty) besides the strictly defined data you need to do your "real work"?
A couple of jobs ago I worked at a financial services firm with 2 networks and 2 PCs on everyone's desk. Rednet for outside connectivity, and an internal network for real work, and never the twain shall meet.
NO-ONE needs the Internet for real work, let's be honest, just for goofing off. Time we all started to prioritize security over mere convenience.
Let's say you're a P.I., journalist, researcher, law enforcement, or intel agency, and need to automate news or people searches for some reason. If you were able to very strictly define the data you're expecting to receive, isn't there any way you could automatically pass this data on to a secure system without opening yourself up to exploits?