Adventurers are already "special" because a player is controlling their actions. NPCs are controlled by their (fairly simplistic) programming and it's a fine line to say whether an NPC is unable to do something or simply won't do it because they aren't programmed to. You aren't going to force a player to perfectly imitate the behaviors of an NPC, are you?
The distraction system was implemented as a compromise. An NPC who likes new clothes will go out and find them, and wear them if possible, because this is what they are programmed to do. The player is not forced to do so, but if they don't they get a distraction debuff. A peaceful NPC will never randomly murder an innocent bystander for no reason, because they aren't programmed to. The player is allowed to but will be warned first and experience appropriate consequences if they do so. As the player, you can do whatever you want within the bounds of the game's physics engine, but stray too far from doing what your character would do, and you suffer penalties. This is how the game works in all respects except for cannibalism.
There's no logical reason why mass murder should be only discouraged with a warning, while cannibalism should be completely forbidden. If you think otherwise feel free to say why.
As for the argument that it "doesn't solve any problems", consider the following: If I'm out in the tundra with the corpse of a recently-killed cyclops, and I'm not quite starving yet but I'm hungry and several days away from civilization or other reliable food sources, then I have the following choices: drag the corpse with me for miles, slowing me down until the game decides I'm starving enough to eat sapients, wait around until I'm starving, or just eat the body now and deal with the distraction consequences for going against my character's moral code. Inventory/weight management is a pretty big part of DF adventuring, so this is a choice that can come up on a fairly regular basis.
Interestingly the game doesn't actually stop you from eating sapient meat, it stops you from butchering the corpses and using their parts as regents. (If you find sapient meat in a night creature lair you can eat it). This also has some pretty silly gameplay ramifications - if you want to prove that you've killed someone you can show people a part of their corpse, but if you killed them without chopping off something small like a head or a hand your only option is to drag the whole corpse with you, weighing you down. If it's a sapient monster like a cyclops or a giant, this becomes ridiculous.