I've had more than one beginner on a jump who flipped my fuel selector on the way to the door. It was more of a panicked attempt to hold on to something than anything else. I started putting my hand on the selector when we had tandem or new guys jumping.
I wasn't worried about making a dead stick landing from over the top of the field @ 12,000' if some dude stole my mag keys, but the owner of the drop zone would have kicked the guys butt for shock cooling his engine that I can promise you. Most drop zone operators were paranoid and for good reason about shock cooled cylinders. Those things are expensive!
Of course a pilot in the know would simply reach up behind the panel and pull the wires out of the mag switch which equals mags hot, which equal engine running.
Just in case it ever happens to you.