How are they supposed to know that? The guy didn't call 911 and say "I'm going to do suicide by cop!" He pretended to be a concerned citizen calling about someone with a knife. If you read the article that Sinobas linked, it says:
"A student's cellphone video captured from a dorm room across the street shows the confrontation play out in the brightly-lit entrance to a parking garage. At least two officers have their guns drawn as Schultz walks toward them with their arms down.
A knife is not visible in the cellphone video seen by CNN.
"Come on, man, drop the knife," one officer says. "Come on, let's drop it," another officer says.
Schultz walks toward them slowly and shouts, "Shoot me!"
"No, drop the knife," the first officer says.
The officers repeatedly tell Schultz to drop the knife and one says, "Nobody wants to hurt you." Another says, "What's going on, man?"
More officers' voices are heard, telling Schultz not to move and to drop the knife. Schultz pauses briefly, then takes three steps forward before being shot once and falling to the ground.
Schultz died at a hospital."
Look how close he is to the cop. That's WAAAAAY closer than 21 feet. That's more like 10 feet, and he was closing the distance.
So let's say you're a cop, you get a call about someone potentially having a knife, you show up and encounter a guy... in the moment..... with something in his hand. He's moving towards you. You're begging him to stop advancing but he isn't listening to you. What do you do? You can't hit the pause button. You can't run away because he has a knife and he's already extremely close.
This is one of those situations that's easy to monday morning quarterback, but in the heat of the moment it's not nearly as cut and dry.