I really wondered what would make an average detective think he would have a chance one-on-one with an arch-criminal. Was this scene to indicate that the Joker's formidableness was largely unknown or was it just used to make the plot move forward?
It is clear that everyone thought the Joker was a physically unthreatening clown -- even the pimp who wanted to "teach him some manners" before killing him thought this even though he had witnessed him killing one of his bodyguards in the blink of an eye, like a magic trick. This is calculated. So maybe a better question is, did Batman himself underestimate The Joker, failing to tell the police just what they had on their hands? This movie established The Joker as a true criminal genius and even unarmed one of the most dangerous men alive. (Note: If the police had captured Bane, they sure were not going to leave him in a room alone with one detective, seated at a table wearing ordinary handcuffs.)