Like many others I don't do that.

Firstly it's low grade bullying. That may not be how you mean it, but you're not on the receiving end. Bullies often don't think they're bullies.

Secondly it normalises a third party using your computer. "Oh I was just pranking Jim" - sure, but were you? Or were you also granting yourself some privileges using Jim's authority? Who knows.

If you have zero tolerance for unlocked machines you should have zero tolerance for this too.

Yes, it's not that big a deal, and yes, I'm the kind of spoilsport who doesn't really like pranks at all. But still, cut it out.