The issue with stopping putting these systems on the internet is that they have been installed for a number of years prior to the internet having so many nasty people attached to it...

Removing them now and placing them onto private networks would have devestating effects involving downtime and possibilities of the actual system to be screwed up..

Yes nowadays the newer SCADA systems are being placed behind air-gapped networks and the like but you still have the problem of moving data backwards and forwards for review so you do have to allow trusts or the use of USB keys to be moved backwards and forwards. Having been involved in the design and security RMADS of these networks it is a ball ache.

SCADA systems are not just limited to the 27/7 manned industrial companies and energy suppliers for instance, they are used by unmanned IT support offshore oil rigs which require support from the mainland. This is typically supplied by sattelite. It has also been known to be done via GPRS using a mobile phone attached to a PC. It is also worth pointing out that the likes of Roll's Royce monitor plane engines during flight (Emirates) and can make changes to the engine in terms of performance for instance (a bit like a F1 car) all done on the fly (hehehehe).

Alot of companies do not see two PC's on a desk attached to different networks as acceptable anymore. Alot of people like using one PC, one screen.