The aim, as you might suspect, is to game the system. In addition to skewing reviews, the bribery gives merchants an illegal advantage by letting them know customers' shopping habits, popular keywords and other details that would normally remain secret. They can then rewrite their product descriptions and ads to increase the chances of their products rising to the top of search results.
Amazon has reportedly taken steps to counter this kind of gaming, including a recent executive swap in China to "root out" bribery. It might face an uphill battle, however. The internet giant's increasing volume of third-party merchants helps it sell a wider selection of products and keep prices down, but it also leads to cutthroat competition where some companies are willing to break the law to get an edge. And when relative income tends to be lower in China, Amazon workers can be tempted to take bribes they'd otherwise turn down. Amazon may not fully address the issue unless it can tackle the incentives for leaking data, not just the leaks themselves.
Update: Amazon has provided the same statement it gave to the WSJ, noting that it has "zero tolerance" for this behavior and has limits on data access to minimize these sorts of incidents. You can read the statement below.
"We have strict policies and a Code of Business Conduct & Ethics in place for our employees. We implement sophisticated systems to restrict and audit access to information. We hold our employees to a high ethical standard and anyone in violation of our Code faces discipline, including termination and potential legal and criminal penalties. In addition, we have zero tolerance for abuse of our systems and if we find bad actors who have engaged in this behavior, we will take swift action against them, including terminating their selling accounts, deleting reviews, withholding funds, and taking legal action. We are conducting a thorough investigation of these claims."