With all due respect, the police ALWAYS have an excuse,[...]
As long as their actions can be perceived as occuring within guidelines set forth in the rules of
Procedure, you are quite correct.
Those who watch the popular "ride-along" police documentary,
COPS, will occasionally see examples of excessive force being used in a manner which conforms with
Procedure. I have seen more than one example of an arrest subject who is offering no resistance whatsoever being forcibly thrown to the ground, his face pressed forcibly onto a filthy street by a heavy policeman's knee, another policeman's knee driven into his spine, and his arms twisted with unnecessary force while applying handcuffs. This sometimes happens while the arrest subject is screaming in pain and the police are simultaneously shouting
"Stop resisting!" for the benefit of the camera and/or any observing bystanders.
Because it is virtually impossible to prove a subject was not in fact resisting, this common example makes it clear how established
Procedure enables police (who are so inclined) to effectively brutalize an arrest subject while remaining well within the rules.