Even if it isn't an intentional attack you don't want people bringing God knows what on USB sticks that may or may not just be infected from the users own home PC. USB storage devices are lovely targets.

But yeah the South Korean military got infected by a soldier plugging in a planted USB stick.

I think the narrative of a targeted attack is easier to sell though. Make it us vs them and people grasp the concept a little better. This is very common in information security training in a lot of fields in my experience.