It's not that they're a monopoly, although they are, it's that they are a natural monopoly, which doesn't require government interference to exist (as a monopoly). If it did, then Bing would be the dominant search engine.
Now there are generally many possible sources for any news story, and Google can choose whichever it wants to choose. If it has to pay it would probably pick AP, Reuters, maybe a couple of others and ignore the rest. Whoops! There go the local news sites. How many people will go to a site that promotes the local high school soccer team? A few. What will the advertisers pay? Not enough to run the site, so it will depend on someone doing it as a hobby. How many sites will be able to pay for an AP and Reuters connection if they aren't indexed by Google? Not many.
So you have a natural monopoly. And making them want to stop indexing you is a fast route to bankruptcy.
Please note that these same arguments apply if you substitute a different search engine for Google. ANY other search engine.