I was thinking that they are supposed to pay you, or you have to pay, depending on how much damage you've done, and that the bug is that you have to put at least one piece of your own in (gear, 1 gold), in order to make it work. However, you could be right that is an exploit and they aren't supposed to pay you at all. I'm really not sure.I couldn't be a kingdom yet (rank 2) and I'm not a vassal. That's why I consider it a possible exploit that they would pay me for peace.
I wonder if they use a in complete war-score to determine peace, like they just see your victories on them and judge only by that.