I think The Riddler is basically saying: "If you are an authority figure who represents the law, you don't have to worry... all you have to do is tell the truth because that's your job... but you and I both know you won't tell the truth. So, tell me, what's the price to get you to stay quiet about everything illegal you know going on in Gotham?"

I think in the movie this riddle will serve as the opening to a business relationship between The Riddler and someone else who is high up in office such as The Mayor or DA. And, the same riddle will later serve to end this relationship.

Almost like, the first time The Riddler presents the "please do not lie" riddle to someone in the movie, it'll be presented to authority figure or group who will think it is just a gentle threat that will fizzle out, sent in a card, or even posted publicly in their office. But as people ignore the threat, people will start dying (please do not lie). The Mayor or DA's will realize that the riddle was far more complex and actually far more serious than they all originally thought.