Obviously doing it yourself would but they probably put it in the DMG to remind the DM since that's the person that tends to forget this fact more often than not.Putting it in the DMG doesn't help much when it comes to showing players that not all fights have to go to the death and that fleeing is sometimes wise.
I've always played my NPC's as if they don't want to die. It's funny since the majority of my NPC's don't even kill when they down PC's since that sort of stuff just increases escalation and increases the odds that they themselves will be killed if they lose.
If my NPC is good-aligned, they may even use their action to stabilize (but not awaken) the PC they've just downed. Neutral NPC's might or might not. Everyone also knows that a murder charge isn't worth whatever they're doing.