I also think it's situational.
Usually, the NPCS are trying to achieve something, which is often surviving being attacked by Team Sociopath as they are just doing their 9-to-5 job of guarding the necromancer's tower door. They will priorize removing the threat and focus on active Sociopaths before finishing unconscious ones doing their death saves. In some cases (ghouls have been given as an excellent example) they will take a sample of flesh because they are unintelligent. It is telegraphed. They might also be trying to feed, and in that case, they will just flee with the prey if they can instead of continuing the fight (usually Team Sociopath will block that plan).
On the other hand, as soon as someone raises from unconsciousness and resume fights, usually through the use of a healing ability, then it makes sense to finish the wounded off. I don't have many opponents assume healing abilities in a low-magic world, but in say Eberron, where most of the population exits from a century-long magic-rich war, "finish them off before the battle medic can get to them" and restore their fighting ability to full" is a common command from officers.