Could it be like a fight or die sort of situation with some of these ISIS militants? I'm not too familiar with their situation in particular, but I would imagine it's like living under any sort of crazy totalitarian style government. They're given the "choice" of pledging allegiance or probably getting themselves and/or their family killed/imprisoned, similar to North Korea. Sure, some of them most likely were/are strong supporters but I find it hard to imagine that no one in that region found what they were doing reprehensible. Despite all the anti-West propaganda, I like to believe that there are people living in ISIS controlled regions who can see through all their bullshit but have to remain silent. On the other hand, it could just boil down to a preference for more humane treatment in the face of obvious defeat like you were saying.
According to Kurdish officials, they put up no fight at all, other than planting bombs and booby traps.
One of the men smelled so bad that when he was taken into the small interrogation room, those inside were startled.
From what the article says, it doesn't appear the members making up the ground troops of ISIS believe that their ideology is worth dying for, as ISIS likes to project. It also seems they were living in pretty squalor conditions and who knows if they even had enough supplies to survive in reasonably comfortable conditions. I mean, it looks like they were in a shit situation fighting for ISIS (duh) so I don't see how any reasonable person would willingly sign up for that unless coerced or led to believe all the lies and propaganda spread by ISIS.
Fear is a strong motivator; self-preservation is innate. The very real threat of death, torture, starvation, etc. for yourself and your family makes both scenarios plausible I think. I'm not excusing their actions whatsoever by the way. No doubt the vast majority, if not all, of those that surrendered have committed heinous acts and should be appropriately punished, but I digress. I'm just trying to gain insight on this situation as a whole I suppose and be more informed on current events.