I have PLC's here that only have 32K of RAM, so there isn't a whole lot of space to load the full TCP/IP stack, they also don't have powerful processors. Nmap scans can bring these things down almost instantly, and doing that kind of thing in an unknown environment can be disastrous. You have no way of knowing how these devices are going to fail. If the PLC programming isn't written correctly, it could fail to a very dangerous state. Equipment could start or stop without warning, it could run things to the wrong speed, a plethora of bad things could happen.
Plus, it is not expected that the company's IT department should know much about the control network. IT is generally not in charge of running the control networks, many times those things are setup by System Integrators that have trained Process Control Engineers. The systems are configured and then left to run. People that don't know their configuration shouldn't be mucking around in them.
There is a current movement to have IT and PC work together and cross train so IT understands process control and PC understands information technology. It is not unusual for IT to not know what the control network is doing. It is also not unusual for management to tell IT that they need access to the control network to get metrics and whatever else management needs to fill their time. This normally leads to bad things happening.
I had a person on site that was from a system integrator, he attempted to do a portscan of my control network. Fortunately, my system caught the attempt and killed it before he had a chance to execute it. But if he had, he probably would have taken down 9 PLC's and the entire plant. I told him to leave immediately and I did not want to see him at my site again. He was then fired from his job just for the potential of causing damage.
One more point, if you see a fancy display that indicates really cool controls of really big machinery. Never think for an instant that they have state of the art equipment. The attached devices out there are probably old, the current cycle for control hardware is around 20 to 30 years, whereas IT lifetime cycles is 3 to 5 years.
Never ever mess around inside a control network unless you're personally are ready to deal with the extreme consequences of millions of dollars lost, hours of lost work time, and the potential of injuring or killing someone.