Maybe they saw how well it worked out for George Zimmerman.
Looks like part of the plan to societal self destruct.
Why, they learned a lesson from GZ's misadventure. They are not going to call the cops, neither before the confrontation nor after. It is an old maxim that involving the police can only make a bad situation worse.
If we imagine that GZ and TM were all alone in this world, with no police, GZ would have crawled from under the body of TM, dusted himself off, and proceeded to wherever he was going. Being a good neighbor, he would also arrange for the dead body to be removed. That's all. Total expense: $1 for the spent round, and a few more for a garbage truck run.
Of course, today we do not permit the strong to make their own laws and to enforce them - for better or for worse. But the role of police in GZ's case is hardly positive. They didn't save him from near death, and they are about to destroy his life. They are just finishing the job that TM started.