I used to work at a foundry that had a Rockwell SCADA system. It operated on a completely physically separate network from the normal, internet-facing corporate network. If somebody needed access to both the SCADA system and their email, they had two computers on their desk.
For something not as critical as say, a utility, I think this was a bit of overkill (at least they could have used VLANs), but why is this not a semi-common practice? Why do these controls need to be on a network with a route to the internet?