If I'm parsing that correctly you are not talking about initial reactions but what happens after a character (NPC or PC) does something, such as "tries to intimidate".
But, logically, I don't know why we should separate action declarations from "undeclared" actions. If NPCs have a reaction roll, they must be reacting to something, even if no players have declared actions. E.g., what the PCs look like, what they are carrying, their body language, etc. Also any intrinsic stereotypes/prejudices.*
Wouldn't all that be true for the PCs as well? As a player I might want to kill the monsters and take their stuff, but perhaps my character would respond differently. Why shouldn't I use the dice to see how my character would respond?
*My next character is going to put a "Murderhobo Lives Matter" sticker on his shield. Just because I'm covered in blood and my backpack has looted weapons sticking out of it doesn't make me a bad person. I'm tired of monsters reacting as if I am.