It's a preventative measure, not a panicked reaction. That's why greater than 1 out of every 4 Americans are obese, because Americans are known for being prepared.

Or maybe it's a knee jerk response because some home doctor found on Google that it can be effective in passing radiation harmlessly, combined with yellow journalism and typical American overreaction to media-concocted panic, with a tad of good old fashioned stupidity, exploitation by modern day snake oil salesmen, and the American belief that there's always some overly easy way to solve difficult problems, especially if it doesn't disrupt the way of life that person wants to conduct (see earlier comment about obesity).