The concern isn't that he'll release the information you intend to be released — it's that he'll release information about you. If you leak proof that the government is spraying chemtrails to reduce the excess population, you still might not want people knowing you're the one who leaked it. So the question becomes, would you trust Murdoch to keep your identity secret?

Can we use a less offensive example, please?

Usually the source wants his identity protected, too. Sometimes from the government.