A third-party site who wants to show ads in their own search results could partner with Google to include AdSense ads in their own results. The Google contract originally required exclusivity, and then was changed to require Google ads to be more prominent.
In the context of the market for third-party sites running search ads, the $1.5 billion fine seems extremely high.
On the one hand, it’s not immediately obvious to take a conglomorate’s total worldwide revenue into account to value a fine for a specific market, versus considering only the revenue for that market.
However in this case Google is squeezing out companies like Yahoo and Bing from increasing their own ad inventory in the few ways that are available to them, aside from increasing their own organic search traffic. So this is where the anti-competitive behavior comes in, and perhaps why the fine is so massive.
I can’t imagine keyword search on third-party sites is responsible for a significant share of overall AdSense revenue, I wonder if it’s even a single digit percentage of their overall ad inventory?