It's not like that. USB drives are a popular vector for transfering malware both ways. Which means a perfectly good dead drop can become infected when someone who didn't know he had malware plugs his computer in. USB dead drops are not like cars - they're like a bottle of juice chained to a wall, that anyone can drink from and refill it with whatever they want. You don't have to assume malice to expect such a bottle to be a health risk - not everyone who deposits a disease knows he is ill.

Of course I'm joking with my approval for installing boobytrapped dead drops - but the point is, connecting to a random thumbdrive sticking out of a wall sounds like a dumbest computer-related idea ever.