Hacking Critical Infrastructure

Hacking Critical Infrastructure:
A otherwise uninteresting article on Internet threats to public infrastructure contains this paragraph:

At a closed-door briefing, the senators were shown how a power company employee could derail the New York City electrical grid by clicking on an e-mail attachment sent by a hacker, and how an attack during a heat wave could have a cascading impact that would lead to deaths and cost the nation billions of dollars.

Why isn't the obvious solution to this to take those critical electrical grid computers off the public Internet?