There's a fourth class of malicious USB keys: those which discharge -100V across the USB data lines, aka "usb killers". Drop a couple of these on a huge parking lot to create a boatload of damage.
Also, if you are talking about harming individual users, and you are the kind of person who would drop "killer" USB drives on a parking lot, how is that any different from just keying every car? You get no benefit, it only causes damage, it takes near zero sophistication and the end results is it costs people a bunch of money and police start an investigation for vandalism.
People will notice that some dude is walking around and keying cars. If you happen to come across a car owner, you might be in for some pretty violent payback (at least that's what I'd do with someone keying my car).
Dropping USB keys, on the other hand, won't cause any suspicion.