If pitching moment creates drag, how can high and low moment airfoils have almost identical drag polars?
A high Cm airfoil does not mean it will be draggy by itself. However, it does mean that when combined with trim devices (e.g. tailplane, elevator, canard, elevon), the overall drag may be higher due to the trim drag component. 

Since 25% MAC, about which pitching moments are generally cited, generally gives around 20% static margin for low speed aircraft, it's a rough first indication of how much trim would be required.