Why are power grids on the Internet anyways?

With the most recent talks by the CIA about how the power grids are being attacked by hackers, I really wonder if this is just a cover-up for incompetency or if it really exists. The reasoning pretty simple.
A while back, the government went through a demonstration video that was showing how a cyberattack could potentially take down a power generator (dramatized obviously). But a really obvious light bulb popped up then, and here it stands now again.
Why are power grid computers on the Internet?
I mean, here’s the deal. Being that it should be a secure facility, you shouldn’t be able to access the power grids any more than you can access parts of the Defense and military networks since they’re on closed networks. There isn’t outside access, and most of that stuff top secret, and I’m sure clearance based. Which means you would have to physically breach for access.
So the obvious solution here? Pull Net access. I mean, it sounds that simple, but why not? Secure them on a private network like everything else that’s important is on. Billing and actual utilities control can be on completely separate networks with a internal firewall, or multiple security entries such as time coded passwords. Seriously. Why do we even put them online? I can’t see an obvious reason at all so I want someone to enlighten me.
Photo Credit: (OZinOH)