I guess I just don't understand why this would all be done "on the internet".
It all comes down to the almighty buck. Standard internet access is widely available and much cheaper than leased lines or VPNs. Using internet connectivity allows whole "modules" of hardware and software to be "cut and pasted" into new products, as opposed to having to write dedicated code for a proprietary system.
Forbes magazine had a recent article on the vulnerabilities of SCADA systems. An interesting read, if a bit light on technical details (intended for the Wall St. crowd, not people who actually have a clue):
http://www.forbes.com/2007/08/22/scada-hackers-infrastructure-tech-security-cx_ag_0822hack.html
