I think the concern is that the AI is making life or death judgements against people. Some may of course be lawful combatants under the rules that govern such things, but the fact that an AI is drawing these conclusions that humans act on is the shocking part.

I doubt an artillery system using machine learning to correct its trajectory and get better accuracy would be controversial, since the AI in that case is just controlling the path of a shell that an operator has determined needs to hit a target decided upon by humans.