Were you specifically looking at the "News+" tab in the News app? As far as I can recall nothing in there should have ads -- WSJ has some annoying boxes asking you to subscribe with them that are quite close to ads, but they aren't 3rd party advertising at least. By default I think the app will take you to the "Today" tab, which presents recent news from News+ sources and ad-supported sources alike.

Apple definitely needs to do a better job communicating this, it's not at all obvious from the description of the News+ subscription.