Exactly. And it's so hard to quantify what a news service generates. It's not just the advertising dollars at that time - and they'd still get some of that if they showed something else anyway.
A news service brings viewers who are paying attention and will see promos for other shows, and I turn watch them (same with other high rating shows). It also connects the station to the community. Particularly in regional areas, news travels if someone is going to be in a news report, and people will tune in. It creates an active workforce that aren't just selling advertising and will be seen going about their job around town. It makes the network look like it cares about the community, and that is a major draw card for getting advertising dollars in regional areas where a lot of advertising is from local small businesses. A good news service could therefore bring in more money even during off peak times where rating numbers haven't even changed.