No, there isn't. You are not missing anything.
The rules are very clear - you can only choose to deal a non-lethal blow with a melee attack.
The way I reason is that while you can easily control a melee attack, say, by hitting with the pommel of the sword, it is quite hard to control an arrow being thrown at an almost-dead character in order to not kill it. Even if you think something along the lines of "hitting his legs", the fact that he is low HP (enough for 1 attack to kill him) means he's already heavily injured (or he is very squishy to begin with), which probably means even the blood loss from the arrow piercing the leg would be lethal.
Obviously this is not completely accurate to real life, but we are in a world with Fireballs, so... yeah.
I don't see any problem with house-ruling the other way, though
I have allowed many times for my ranged player characters to deal non-lethal blows, sometimes by telling them the AC would be increased (since it would require more precision), sometimes by simply allowing it. I don't see how it would be a problem in any way, and never had any kind of problem following it. I may also ask them to actually perform medicine and stabilize the enemy character in these cases, so he won't bleed to death.
Usually all of this comes as just improvisation and I go along what will make the players more satisfied, honestly. As I said, I never had any kind of problem allowing it.
The rules do allow the DM to use Death Saving Throws rules for NPCs
So, you can always use that. The NPC will not instantly die, it has to make some death saves. The characters can then work to stabilize or even heal the knocked NPC. Usually, using Death saves for NPCs is reserved to important and powerful NPCs, but nothing is stopping you from using it to random thugs.