“Politically neutral" is a phrase used to distance oneself from the largely American habit of slotting one's values into the red vs. blue dichotomy. One can't be truly politically neutral, as you say, given that politics is a system of dealing with others and not an electric charge. However, it's a phrase that is easier to invoke just as it’s easier to describe oneself as an atheist rather than an agnostic when explaining why one would not believe in God.