A couple of ideas:
(1) The centrality of markets to libertarian thinking. Anyone who knows enough economics to even bother identifying as a libertarian should be fairly unlikely to get questions like this wrong -- questions that don't go at all beyond an Econ 101 grasp of supply-and-demand and marginal utility.
(2) The lack of an explicitly libertarian option as a major party in US politics. Because of that, not many people really default to self-identifying as libertarian. Thus, libertarians are self-selected for relatively high levels of political and economic knowledge.
(I've done my best to phrase these as neutral statements and not arguments on the merits of any political position -- thus as "LW-appropriate answers"!)
I think these are good ideas. But still, it's "the third largest and fastest growing political party in the United States" (according to 'Kipedia)! As such, I'd be surprised if libertarians in general could apply econ-101-level reasoning across the board, even in support of "enemy soldiers".
A article in the Atlantic, linked to by someone on the unofficial LW IRC channel caught my eye. Nothing all that new for LessWrong readers, but still it is good to see any mention of such biases in mainstream media.
I break here to comment that I don't see why we would expect this to be so given the reality of academia.