Sylvia, a poet, quotes a line from a poem by T.S. Eliot in one of her own poems. The fair use privilege is perhaps the most significant limitation on a copyright owner's exclusive rights. If you write or publish, you need a basic understanding of what is and is not fair use. In most other situations, copying is not legally a fair use. Without an author's permission, such a use violates the author's copyright.
Violations often occur when the use is motivated primarily by a desire for commercial gain.
The fact that a work is published primarily for private commercial gain weighs against a finding of fair use. A commercial motive doesn't always disqualify someone from claiming a fair use. A use that benefits the public can qualify as a fair use, even if it makes money for the user. The ad significantly increased the number of people exposed to the Consumers Union's evaluations and thereby disseminated helpful consumer information. The same rationale probably applies to the widespread practice of quoting from favorable reviews in advertisements for books, films and plays.