The practical effect of such a law would be to boost traffic for the big-name publishers (who'd know how to opt in) at the expense of smaller bloggers and publishers (who might not understand how to opt in or might not have an easy way of doing so unless Google and other search engines built "opt in" into their Webmaster tools).
It seems to me that this law would give Google--the leading search engine--a perfect justification for PFI (pay for inclusion). Google could impose a sliding scale of service fees for German publishers who wanted to be included in its Google News results, starting at zero for very small publishers, bloggers, etc. and rising to, say, 100,000 euros a month for megapublishers like Springer. Google could justify this by saying that it would help to level the playing field while protecting free access to a wide variety of news sites and blogs by the German public.