because they're using someone elses intellectual property in the process, and that's what this law is about. Sure, the relationship goes both ways in that it increases exposure for the newspaper, but there is a very principled case to be made that Google profits for free off the intellectual creations of third parties.
The one point I agree with in your question is "why only newspapers", I think we should systematically move towards monetisation when Google indexes, reuses and displays content from third parties and incorporates them in their products.
Also there is no genuine search competition anyway, so instead of waiting another 20 years I'd just be happy if Google starts paying people for the content that they use.
That would be a terrible business model though. Maybe the French newspapers should join together to create a search engine of their own content, if there really is a lot of money to be made in that game.
I think we need to pass legislation that prohibit exclusive content licensing and require that equivalent licensing terms are offered to all competitors (similar perhaps to rules about patent licensing when it is incorporated into a standard?)
Edit: Since Google has a near monopoly on ad networks, there is a sense in which Google already is the one paying for much of this content.
This is using law to grant the lobby of newspapers additional revenue. And blame Google for not abiding.