Cell towers are generally going to be actively defended though - they tend to connect to private backhaul circuits, or link by IPsec to the security gateway in

Cell towers are generally going to be actively defended though - they tend to connect to private backhaul circuits, or link by IPsec to the security gateway in the mobile network. The difference here is that nobody takes responsibility for a water treatment works in the same way a mobile operator looks after base stations - most operators aren't putting their base stations anywhere near the public internet. When they do it's under very careful control, like with femtocells.