You don't need to buy the books in order to pass the certification exams. Experience is often far more useful. A list of topics covered by the exam is often provided, too. So somebody with experience can often easily supplement their existing knowledge by reading some online articles or documentation, without ever looking at one of the official books.
And one of the core aspects of certifications is that they usually very specifically target a given product or topic. This is one of the things that differentiates them from college degrees and other ways of suggesting qualification. A certification can help an employer gauge a candidate's abilities in a far more specific manner than a Comp. Sci. degree can, for example.
Certification may not always have the value or reliability that it's claimed to have, but let's not misrepresent it or its process out of ignorance, either.