Perhaps terrorists with camping skills have a higher threat potential? It's not just about P(terrorism | camping) - they also care about the expected value of terrorism plots involving particular suspects.
Like, I'm not even sure that terrorists are more likely to be known to own guns. The EV of terrorist plots involving gun ownership are higher, though, so more investigation ought to go there.
You make a great point about expected value of damage done, rather than focusing on the probability of an attack. But I think an attack with guns has a low upper limit on the number of people you can kill (maybe around 100, plus or minus?) before an armed response is mobilized. Things like nuclear, chemical, biological attacks or bombs bringing down buildings scare me more from a statistical standpoint.
I'm pretty sure that P(bombs|guns) > P(bombs), so my point stands. It's a mix of guns being dangerous, as well as gun possession being evidence of possession of other dangerous things.
