According to the Federal Trade Commission, bloggers have to disclose any “material connections” between themselves and any products or services they post about.
Reviewers posting on Floccinaucical.com may have received a galley, ARC, or copy of a book or cd or other product they are reviewing from the author, the author’s PR person, or the publisher or producer. They may also have purchased the book or CD or other item, or borrowed it from the library or received it as a Hanukkah gift or won it by entering an online giveaway.
It’s probably safe to assume that someone somewhere has a copy of the book or CD or product they didn’t pay for and got to keep as remuneration.
But it’s also safe to assume that the review you’re reading represents that person’s honest opinion.