In a way he does buy advertising.
Advertisers for the news shows he is on pay a LOT more for their time than when he's not there.
25+mil watched the first debate. Just wait until the General Election.
What you say is true, but I’m not sure the enemedia looks at it that way. They think viewers tune in to see their clever and witty talking heads engaging with the candidates. Their talking heads are the stars of the show and the candidates are merely the props.