When was the last time someone getting released from prison early on a manslaughter conviction made the news? Perhaps, just perhaps, it happens all the time. Perhaps, since it doesn't raise an eyebrow or make the news unless it's a high profile case or involves a police officer, the perception is that this officer is receiving treatment different than what many, if not most other convicts receive.
Thousands of people receive light sentences or have their charges dismissed before trial every month in this country. Proving guilt beyond a reasonable doubt is not an easy thing to do (for good reason). No one notices. As soon as one of those thousands is a police officer, prior police officer, someone who lives next door to a police officer or played cops and robbers as a kid, it's a huge cover-up/conspiracy.