Byrneseyeview is presenting an overly simplistic view. It's true that we do respond to stimuli, but fortunately we don't respond only to stimuli but also to our internal states (thoughts, feelings, memories, and even moral judgments). As a result, I don't think that physicalism is incompatible with morality (even if it is incompatible with free will) because you can still interpret someone's actions in the context of their mental state--just like the legal system currently does.