Two uniformed men strolled into the main room of the Little Falls library in Bethesda one day last week and demanded the attention of all patrons using the computers. Then they made their announcement: The viewing of Internet pornography was forbidden. The men looked stern and wore baseball caps emblazoned with the words "Homeland Security." The bizarre scene unfolded Feb. 9, leaving some residents confused and forcing county officials to explain how employees assigned to protect county buildings against terrorists came to see it as their job to police the viewing of pornography. After the two men made their announcement,...