FOR A PEOPLE so ready to cast off ancient customs, Americans have been strangely reluctant to question the practice of taxation. Begun millennia ago, taxation developed as a method of exploitation. Instead of massacring defeated peoples, or taking them as slaves, conquerors realized that they could make a regular income by demanding wealth--with the threat of massacre as a prod. Of course the methods of taxation have been refined since those early days, but they still rest on the ugly foundation of force and the threat of force.