Because of the principal of the fleeing dangerous felon. When you have a person who has just committed a dangerous felony that has escaped from the police, they pose a very, very real danger to the surrounding community. They are fearful, they have just committed something that they will go away for a long, long time for doing. Who knows what they will do to escape. Hold up some nice family in their home and demand an escape plan? Steal a vehicle and drive recklessly down the freeway? We do not know. That is why we have the doctrine that police may use deadly force if there is a fleeing felon that poses a danger to the community.
That danger is heightened when it is a juvenile who doesn't have the same risk balancing skills as an adult felon.
I said if the accounts are true. I do not know if they are true, and neither do you. It may or may not be true. But if true, the cops were justified. If not true, screw the cops.