Apostates want to surrender. They are apostates because they want to give it up. A Moslem dies in the fight against infidels. One who does not want to die fighting the infidel is ipso facto apostate and deserving of the sort of death that closes the gates of Paradise to them.
Motivational reasoning... more drastic than the mere decimation (10% reduction) used by the Romans in the waning days of the Roman Empire.
There have been several reports of unpleasant executions of retreating fighters in the past for exactly the reasons you said. Word gets around.
That is the mindset of the Jihadi’s.
That which has been seen cannot be unseen.
Locals and some “true believers” have seen a lot of what Islamist Terrorists really are. Some are disillusioned.
First instinct? Kill em all.
But it is no my call, only the Kurds and Arabs living there can judge the risk of not eliminating them all.
Kurds are not all Muslim, especially those in Northern Syria. All Arabs are not the same, many in Northern Syria have seen what ISIS Terrorists are and totally reject it.
So, this is a good sign.