I much appreciate the comments in the Electromagnetic Pulse and Smart Guns thread, and I wanted to follow up on one question.

Some commenters -- and others I've talked about this subject -- suggested that it's not sound to make policy decisions based on this sort of speculation about the possibility of an EMP attack. (I set aside for purposes of this post whether one ought to speculate about the availability of smart guns; let's assume that smart guns become available and reliable, as the New Jersey conditional smart gun mandate law presupposes they will be. I also set aside whether smart gun mandates are prohibited by the Constitution even if it turns out they don't materially interfere with people's ability to keep and bear arms for self-defense; that will be the focus of more posts in a few weeks.)

Let me probe this speculation issue a little. The existence of EMP is not speculation: EMP, unlike sex-starved velociraptors, is quite real, as are nuclear bombs, as is EMP that goes much further than the bomb's kill radius. The speculation comes in guessing about the per-year probability that America would be subject to an EMP-generating (but not otherwise immediately lethal) nuclear attack.

But is such speculation really improper -- or even reasonably unavoidable -- when it comes to policy analysis? Imagine that there's a proposal to spend tax money to shield American infrastructure installations against EMP. I assume we wouldn't condemn it as inherently unsound because it's built on speculation about the likelihood of an EMP attack. Of course we could always debate whether it's worth spending the particular amount of money that's proposed, given other possible uses for the money (including lowering taxes as a possible use). But to resolve that debate, either in favor of spending on EMP shielding or against it, we'd have to speculate about the risk of an EMP attack.

Is there some inherent reason that such speculation is (1) proper for evaluating the merits of spending programs, but (2) not proper when evaluating the constitutionality of regulatory programs (in the course of determining whether the programs excessively burden the exercise of constitutional rights)?