re: Dirty bombs are psychological weapons.
Americans have been trained by years of environmentalist fearmongering to be terrified of radiation in any form or dosage. If I were to spread radioactive material barely detectable on a dosimeter around a public area, many people would avoid it for years, no matter how safe authorities said it was.
Anything radioactive enough to be actually deadly while spread widely around an area would, as you say, wipe out the bombmakers pretty quickly.