Not saying it's right. It's how they are trained. They are not required to protect lives (innocent or other), and "officer safety is paramount". They are trained to kill in preference to risking their lives. If he did have a knife and they let him walk away like that, they showed amazing restraint.
I trained with some new cops years ago as a guest of the trainer. There was a session where all we did was stand in front of a target at the range which the trainer could move toward or away from us. If the trainer said, "he has a [weapon]", and the target was within 10', you failed if you didn't fire. You also failed if you didn't fire at least 6 rounds.