In most US states, it is illegal to fail to report a crime. This is usually a misdemeanor, but is a felony in some states and with some types of crime.
I'm gonna go ahead and claim that laws that incur a penalty for not reporting a crime, are not commensurate with legal obligation. Such a charge sounds like a plea bargain or additional-charges or inconvenience-a-witness tool, than a legal concern. The charges for failing to report a crime are subject to prosecution that includes due process (proof, legal prosecution, etc). It's nearly impossible to prove that you witnesses a crime you didn't report without some paper/video/interview trail. This is partly why you never talk to police without a lawyer, as it can get you caught up in such a charge by your own statements.