You shouldn't underestimate how much newspapers know about the economics of their inventory. In many ways, ad slots are price discrimination. Companies that want 100% share of voice (ie: no one else gets ads on the page) will pay a premium to get it. A small company may just buy a cheaper ad slot at the bottom of the page. They have thousands of permutations on how they can sell ads, and they will shut off as many slots as necessary to get a premium campaign going.