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That quote seems to counter the 'nobody knows why' in the article subtitle.

If it backfires when Police can choose to turn them off that seems to suggest that they act as if having a camera protects them from being questioned. Since they'll likely record video evidence in their favor and just turn it off if they want to use force.

In the case with no camera things would be more balanced (though still skewed toward the police officer's testimony), but selective camera use strongly biases favorably to the police officer.

A camera that's always on should help get closer to the truth for the officer and the suspect.