I'd guess that the idea would be that a fairly successful attack with a radiological, biological or chemical weapons would have have a disproportionally large psychological impact compared to a conventional attack with guns and explosives causing an equal number of deaths. The object of terrorism is to cause terror; inflicting casualties is merely a convenient means to that end.

A successful bio attack would cause a paradigm shift. Imagine TSA having to search every boarding passenger for possible biological devices.

Jeff