It's the easy/fast/painless way of doing things. Blaming the players for doing it hardly seems useful.
Yep, I understand completely. I know I've advocated the method myself in the past.
It is important to mention, especially when talking to newer players, that the "kidnap/execute" scenario is not something that is implemented in game mechanics. The other party can't just click a button that says "arrest the other guy". Arresting nobles is something that requires certain specific conditions, such as a war declaration, and even then there is no "arrest a noble" button. (Priests being a special case, and even then requires a war declaration.) And execution requires a certain set of pre-existing conditions.
I believe the situation was such that the Ruler was already banned from the realm they were exchanging regions with. I'm not sure what changes that would make to the situation, but that was where that came from.
Yes it is not a standard case.
