It is usually a result of a fight between control system engineers and IT people. The IT people see an ethernet cable and think that they should control it and

It is usually a result of a fight between control system engineers and IT people. The IT people see an ethernet cable and think that they should control it and in order to do that they connect the two networks. There are ways to do it safely, like placing a database in a DMZ that the PLCs write to and have computers on the other side read from the database without ever connecting to the PLCs, but that requires expertise and a security mindset.