Police Procedure
The other outsider aware of Amy's manipulation is Detective Rhonda Boney, who immediately relies on Nick's opinion the moment it's apparent he's not guilty. Although she fully believes Amy has orchestrated the whole thing she's utterly powerless to do anything about it. The case is now in the hands of the FBI and the presented story is airtight enough to not warrant further question. It's of course required from a narrative stand-point so why Nick didn't just go to the police isn't left as a niggling question, but it also adds to the downbeat compromise of the whole ending. Throughout the film the red tape of modern policing is ever present - the cliché of a hunch is repeatedly mocked for its ridiculousness and a murder case can't be made despite small pieces of evidence racking up - and the ending shows how, while this rigour helped Nick maintain his freedom, it's far from perfect. The downsides of police procedure was something Fincher has previously explored in Zodiac, with various neighbouring police departments poorly coordinated to such an extent that they didn't corroborate evidence on the titular serial killer.