The implication is something out of the ordinary with the reactors is happening or a cause for concern. After all, otherwise it wouldn't be news. So maybe the readers will think the Russians hacked them and shut it down. Or that the plants melted down. News services have an ethical obligation to report accurately and not fear monger. In the last 10 years they have really abandoned this mandate.
"adding to electricity demand on European grid" is also misleading. No more electricity is being demanded since these are offline. We're not turning on millions of additional tea kettles. Electricity production is being reduced. The stress placed on the European grid is increasing because of this, however.
Routine maintenance isn't news. They know this, but news outlets don't write headlines to be the most informative or to give the readers an accurate picture of reality. They write them to get the most views.
"Half of France's nuclear reactors offline for routine maintenance, increasing stress on European grid" is far more informative and accurate but induces less fear and anxiety in the readers, hence gets fewer views.
Sadly people are so used to shitty news service that they don't even recognize it when they see it anymore.
Clickbait/fear-mongering headline.