Police aren't trained to shoot people in the head, they're trained to shoot the "center mass" (i.e. chest area) if they need to shoot someone at all. And the main reason police have fire weapons is when they feel there is an imminent and credible threat to their life/safety, which a robot would eliminate.

I'm not saying robots can't be abused in this application, but I think you can make the argument that if deployed correctly they would result in fewer people being shot in incidents with police.