Sure, there could be some corruption going on, but theres a much more mundane legal reason the DOJ won't bring cases. They often won't prosecute because the costs of losing anti-trust cases are so high, largely because of the way antitrust law is written. When the DOJ brings a case and loses, the agency loses all future power to bring a similar case because precedent is binding, so they only prosecute the most obvious anti-trust violations. As all these conflicting comments in this thread and related articles suggest - an antitrust case against Amazon is quite complex and not your straightforward monopoly or price fixing case.