Because the killings are targeted actions that are arguably justifiable in the face of tyrannical action.

If a story broke about a criminal gang who all wore identifiable colors and claimed the right to stop anyone you saw and bully them to the point of death, you’d demand that effective (violent) action be taken to stop them. But because the gang is “the police” and nominally controlled by elected officials and the courts, there is a public policy reason to treat both their misbehavior and the public reactions thereto as something categorically different.

(I’d be all in for abolishing police costumes and requiring them to act only within the bounds of permissable behavior for the rest of us, FWIW )