Final answer:
Terrorism is mainly viewed by legislatures and policymakers as a concern for national law enforcement agencies due to the specialized resources and coordination they offer for protecting against such complex threats.
Step-by-step explanation:
The perception that terrorism is primarily a concern for national law enforcement agencies such as the FBI, ATF, and DHS is largely based on the nature of the threat and the federal government's responsibility to protect against foreign invasion and domestic uprisings. While local police departments and state police play a significant role in immediate responses and maintaining public order, complex issues such as terrorism often require the specialized resources and coordination that only federal agencies can provide. This is exemplified by legislation like the Patriot Act and the establishment of the Department of Homeland Security, both of which were aimed at enhancing national security following events like the 9/11 terrorist attacks.