Why did I get penalized for this?

[SalemP]

You get a 30-minute penalty for leaving a duty. End of. Doesn't matter if someone left before you. You either have to wait for a replacement, call a vote to abandon the duty (and hope the others vote yes), or leave and suffer the penalty.

Also, I'm pretty sure the game warns you before you leave the duty that you'll get a penalty for leaving and asks if you're sure.