Why are dams, nuclear reactors, power stations, etc. even connected to the internet? I remember watching one of the Matrix movies as a teen and there's a scene in which Trinity hacks into a power station and I laughed mentally and thought there's no way that'd happen IRL. Whoops lol.
AIUI the main dam isn't. The gate that was opened is a small gate on the side, which is operated by someone else. The dam operator manages the big gate for electricity, someone else has a small gate on the side to control water levels downstream.
I would guess that it's on the net because the someone else isn't inside the operator's network.