One who sees no evil, hears no evil, won't be stopping evil.
Some people choose to see no evil and hear no evil. What they do is that they don't see evil as evil. If they believe what they see is not evil, then to them they see no evil. Similarly, if what they don't believe what they are hearing is evil, then to them they hear no evil. From these kinds of people, one thing they'll don't do a lot of is speak no evil.