Different scope, though. I don’t think quarter or no quarter affects the whole country’s willingness to surrender (the chance of that is basically zero either way), but it does affect individuals. If you have a few enemy combatants holed up somewhere, it could cost you a lot of people, ammo and time you’d rather spend somewhere else to take them out. They know they can’t win, and it’s either surrender or die. But if surrender means death, they’ll definitely not surrender. Now you need to eliminate them the hard way.

Of course, doing it that way requires more resources, which I guess is good for business in the military industrial complex.