Your separation of what constitutes 'voluntary' and 'involuntary' action comes from the scientific/mathematical tradition of yes/no, right/wrong, black/white answers. In that context, we can say that the actions of the officers were entirely voluntary.

Unfortunately, those simple answers cannot be found when dealing with human motivation. There we delve into voluntary and involuntary thought.

Our brains work all the time, we are constantly having thoughts. But only a small proprotion of these thoughts are voluntary and intentional. Thoughts such as "It's hot today.", "I could go a coffee." "That person is a looker." are reactions to stimuli and as such, involuntary. Voluntary thoughts are: "That anti-cyclone is moving away west, it'll be cooler and wetter tomorrow.", "I'll have the FairTrade coffee rather than exploit people.", "In another culture, that tall, slender, fair-haired person would be a skinny, washed-out scarecrow."

In between, we have habitual, or conditioned, thoughts. Reactions to reactions. Ideas drilled into us by upbringing, religion, education and job-training. Thus: "I'll put some sun-block on.", "I'll go to to Starbucks.", "He/She is way out of my league."

Most of us are lazy to begin with, and on top of that, true voluntary thinking often requires time, space, quiet and data.

I submit that the officers in question were acting on conditioned thoughts brought about by who they were, where they came from and the job they do. Thus:

"Black males are more aggressive, so the Black kid started it."

"The Black kid is more likely to attack us."

"The Black kid probably has a knife."

"The White kids' parents are better off, and if we cuff him, they'll probably sue us."

Please note that none of these thoughts have any direct bearing on the situation or any necessary relation to truth. They are simply conditioned reactions to a given situation, based on years and generations of conditioning.

I would even go so far as to propose the following scenarios:

If both kids were white, but one was wearing the uniform of a local, highly-regarded Prep School while the other was in t-shirt and jeans, which one would have been handcuffed?

Now suppose the one in the Prep School uniform was Black?

I know how both those would play out here in the UK, but what about in the US?