They're not endorsing candidates in exchange for money. They do use their money to run their networks, which they use to promote certain candidates and positions.
Re: "The line is clear: is money being exchanged in order to promote a product? That's advertising."
What if they are running the story on local ocean tides or soup kitchens? They are doing this to sell more newspapers or more airtime for advertisers.. does this mean there is an "exchange for money" under your rule?
Well, I'd argue that all stories don't fulfill the same purpose, and that such a small story doesn't have enough importance to the broader public for there to be an "exchange for money" of the type I've described.
But also, it seems pretty clear that political stories specifically generate massive cash flow for media, through clicks and "online engagement", the spectacle of debates, video of gaffes, and so on. I'd assume that is why the political "season" lasts longer and longer? The politicians certainly take advantage of this and use it to their ends. The media seem not to care as long as they continue to get "paid", in their way, and have access.
Re: "The line is clear: is money being exchanged in order to promote a product? That's advertising."