The idea is that unlocked screens are a security liability. If someone can read your Slack or send emails, they can download credentials, drop tables, etc.
Firing is a little extreme. We just embarrass people to remind them. Also if it's a safe place – in an office meeting room or something, we're unlikely to do it, but if it's in a co-working space, it's fair game.
For unlocked stuff, it would just be a warning. But if you did any of the prank stuff with an unlocked computer they're talking about, firing is an option (in a regulated industry such as finance).