What obligation? Not a legal obligation, certainly--at least not in the U.S., where the Constitution protects freedom of the press. If your imaginary news agency wants to cover cats instead of earthquakes, or if it wants to skew its reporting to favor cats over dogs, it has a legal right to do so.
Antitrust laws could be used to keep Google from buying up other search engines, directories, and related businesses in the unlikely event that the government could prove that Google was a monopoly. But the First Amendment would prevent the government from regulating how Google analyzes and presents search results.