Actually that's not true. The dispersal area probably would not be very large, so it would best be used in a high value area such as a city. You might get a couple of square miles of serious contamination, but probably not much more.
True,... unless they go after the Central California farming area. California grows 50% of this nations produce last I checked. If they blew up a few dirty bombs there, people would refuse to eat anything to come from that area, even though most of it would be safe. That would be devastating economically. All produce in the nation would instantly double in price, there would be shortages that we would have to augment by using more produce from places like Chile and Mexico with laxer enforcement standards.
They go after say a major water way like the Colorado River or the Mighty Mississippi, even though the danger would be miniscule, people would avoid any activity involving them.
We could go into the second great depression if we don't stop these guys before they act again.
OK, how much nuclear stuff was in Chernobyl???