Bad publicity is still publicity. If the movie adaptation is criticized, some may go to the original book to compare them and praise the book if they think it was better. For example, the movie adaptation of Percy Jackson and the Olympians: The Lightning Thief garnered tons of negative criticism when it was released. Many fans of the book and non-fan moviegoers alike thought the movie was a flop. However, after the movie was released, all five books in the Percy Jackson and the Olympians series were on the USA Today's best seller list in the Top 10.
Thus, even a poor movie adaptation can increase public interest and grow the book's fan base.