There's a gif that humorously demonstrates this mentality [1] and shows a police officer instinctively pepper spraying the air around him in response to falling down, even though there's no one around him.
Personally, I blame the laws & culture that encourage excessive civilian gun ownership. The common excuse for preemptive police brutality is the fear that a civilian could be reaching for a gun which puts the officer's life in danger. Without guns, police have no excuse to strike preemptively and it'd be much easier to gain political support for de-arming the police.