NPCs are Non-Player-Characters in video games, i.e. "extras". Their programming is usually kept simple, so they're kind of famous for robotically repeating the same lines or doing the same actions.

I think here it might mean getting attached to the rules for the rules' sake and forgetting the deeper reason why the rules are there in the first place - like waiting on a red light on a deserted street.

I'm not that commenter, but it seemed clear from the context. Inflexible, decision-tree-like behavior -- like an NPC from a video-game.