Am I missing something? Is this policy a contract that companies have to agree to in order to have their products reviewed by Consumer Reports? If not, then how can CR enforce a policy that a Rating cannot be used in a press release in an advertisement?

If an organization has given you a #1 Rating, that's just a fact, and facts are obviously protected by the First Amendment, in ads or press releases or wherever. You can also use other people's trademarks without their permission as part of a factual statement (e.g. "Coke beats Pepsi").

It is mostly enforced in a completely civil-libertarian manner via protest, bad publicity, boycott, etc. Not by legal liability.