Question about Mercenaries

[JPSeraph]

Why do mercenaries and their troops automatically die if they flee a battle? Wouldn't it seem more reasonable to have them return to your forces if their contract isn't up and there is a safe land to retreat to or to simply return them to the Mercenary cue if the aforementioned conditions are not met?

I really had Pak-Ur going and was disappointed to see that he died because of auto-flee when some single troop that was in the garrison (due to fleeing a nearby battle) died in a battle Pak-Ur could have easily won on his own.