i think should is just a shorthand for 'recommend' -- i.e. if something is recommended, it should be done
Really I can't think of anything getting much more passive aggressive than that.
Honestly I prefer moustache twirling villains who say 'do this or I'll harm you somehow' rather than someone who says 'I recommend this...' and leaves it ambiguous as to whether they threaten or really offer a genuine choice.
The original question on difficulty curves is centered around downrating the game if it doesn't do what it 'should', unless I'm mistaken. It's not a recommendation if you give the recommendation with a gun in your hand (so to speak).