The First Amendment has nothing to do with the issue.

As to whether an opinion is actionable, it depends, as we lawyers always say. If you tell me something and expect me to rely on it and I do rely on it and my reliance is reasonable, I may have a claim for misrepresentation against you. Whether Moodys et al. couches their opinions in asuch a way as to avoid any liability to third parties, I don't know, but this is basic tort and contract law, not a free speech issue.