Lazy prosecution
They went after the corrupt cop using the CFAA, but there are lots of other public graft and obstruction of justice statutes that the prosecutors could have used. Perhaps the issue is that they KNOW that he logged into the computer to run the fake license plate, but the actual mechanics of the corrupt quid-pro-quo that he received for running the plate is probably harder to prove in court.
(I don't remember Sherlock ever having to deal with this stuff. Things were so much simpler back in his day.)