The story goes that a valve manufacturer wanted to have their networked smart valves certified for use in a nuclear reactor plant. The smart valve could report all sorts of critical data points to a database system and also be controlled by that SCADA system in the plant. But to get it certified for use, the smart valve had to undergo a rigorous security audit by the information technology (IT) department at the plant.
Ever notice how emergency exits in a building open outward? Then, to keep the building secure, they are usually locked from the outside. You can get out easily enough, but you can’t get in that way.
However, on a building’s main front entrance, the door often swings in so you can enter. Typically, there is a security guard or perhaps a receptionist there to check your ID and keep an eye on the comings and goings.