They aren't 'on the internet'. They are connected via several layers of networks with firewalls etc in between to a system which has direct access to the internet. There often is no practical way around this - data from these networks needs to be shared with business users, other companies, regulators and so on. Data diodes can be applicable in some situations, but I've never worked with a company that uses one. I don't know the details of this situation, but demanding that it be impossible to compromise infrastructure networks is ridiculous. If you throw enough money and resources at it, no network is secure.