That's false as the DOJ has convicted officers for civil rights violations. In terms of criminal law, that's only tried at the state level.
State vs. Federal Prosecution | Nolo.com
Example: In 1991 in Los Angeles, several police officers were involved in the vicious beating of motorist Rodney King. Because of the case’s tremendous publicity, the court changed venue to Ventura County in Southern California. The Ventura jury acquitted the officers on all but one charge (as to which it hung), sparking the 1992 Los Angeles Riots. Months later, a federal grand jury indicted the officers for the same beating under a law punishing anyone who, acting under governmental authority, violates another person’s federal rights. (18 U.S.C. § 242.) A federal jury ultimately convicted two of the four officers under the U.S. law, which prosecutors frequently invoke in excessive forcecases. (To read about a similar situation, see Federal Charges in the Kelly Thomas Case?)
Please stop posting breh, you're spewing inaccuracies

Was there a state law against it? 
so you really contemplating nikkas words vs Fox News???? you a c00n or a nikka thats severly brainwashed....
Oklahoma