If knowledge of a single murder makes one an accessory, why do you think that knowledge of ten thousand murders doesn’t?
You have an interesting question, so I researched it, keep in mind I am not a lawyer, and I doubt it if you are. One such article said:
“Generally speaking, most people are under no legal obligation to report a crime, whether they knew about it in advance, witnessed its commission, or found out about it after the fact. However, there are exceptions to this law that you ought to know about.”
I could find nothing readily available about simply having knowledge making one an accessory. I did find that if there is a specific law, of which some states have and some federal situations apply, then it may be illegal to have knowledge of a crime and not report it.
I think that is why the thousands of Germans that had knowledge but were not active participants were never charged. Had it been illegal at the time of the Tribunals, there would have been thousands, if not hundreds of thousands of arrests prior to Nuremberg.
To do so now so many decades after the fact when the context of a crime has changed with the liberalizing of the cultures is pure liberal hogwash.