Have you not ever been in a situation where it was plain that everybody was waiting for somebody else to make the first move? Fear is a powerful motivator to do nothing. Fear or losing your job, fear of being stitched up, being accused yourself, losing your own family.
These people are also victims, carrying an immense burden of guilt and shame; doubtless many wanting to act at the time but for various reasons compelled to keep silent. How many I wonder went home and tearfully hugged their own children, and said "nothing" when their kids asked them why they were crying?