It's a very sensible arrangement, really—like, we both know how this whole 'keeping you prisoner' thing is going to go, so why not just skip all the fuss?

I suppose it being a regimented type of agreement, generally respected in wartime, probably plays into the thing where the Jacobites were being very careful to present themselves as legitimate wagers of war, on a level with recognised states, because to them Charles was the rightful king, while the Hanoverians saw them as rebels and therefore not legitimate 'real' enemies.