Yes, national security is a good reason to subsidize food production. I might agree with keeping all those bridges open, on those grounds—though I suspect overall economic efficiency could be improved by removing some fraction of them, without harming rates of food production, and that there's effectively some level of unproductive free-riding going on.

I just object to framing things like urbanites must subsidize rural living because "that's where their food comes from". Sure, of course... but that's why there's a food market. We're not propitiating the farm-gods to shower food upon us, but, instead, paying for food. Subsidies might make sense for several other reasons, but, as someone who spend much of his childhood in the country and sometimes living on small farms, the defensive "but you owe us" attitudes from some people when this topic comes up seem downright bizarre.