Just because we haven't seen anyone die from nuclear terrorism doesn't mean we shouldn't legislate against it. And we do: significant investments have been made into things like roadside nuclear detectors, and during large events we even go so far as to do city-wide nuclear scans from the air to look for emission sources.

That's an "imagined" horror too. Are you suggesting that what we should do instead is just wait for someone to kill N million people and then legislate? Why do you value the incremental economic benefit of this technology over the lives of people we can predictably protect?