Don't let these people on HN mislead you. Even the postal service cannot knowingly deliver a bomb to a target and claim immunity. It doesn't work like that. If you know they are committing a crime, then you have to stop them. Or at a minimum try to stop them.
I agree that a postal carrier would be criminally liable for knowingly delivering a live bomb, but I think you’re wrong on the end part. There is no legal obligation to try to stop crimes you know about. In most cases there isn’t even an obligation to report them.
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.
Same with postal service, etc.