Why was there a detective with the joker in the interrogation room?

Hmmm, it would seem that if they had just left the door closed he would have never got out.

Maybe they were worried somebody would open the door for him since half the force was paid up with the mob. So they closed the door and put a trusted veteran personally inside to make sure he didn't have any miracles up his escape sleeve.

I don't know at the end of the day it was probably just because they had some good dialogue with the joker taunting a police office with how many he has killed and slipping his way out. It was probably just more entertaining than a more realist escape they could have written.

Poor writing. We're supposed to somehow enjoy it as a superhero movie while taking it seriously as a thriller while the cops never wash the Joker's make-up off, the joker somehow survives a bomb by hiding behind a filing cabinet, the joker somehow wires an entire hospital with explosives without been seen, Batman somehow knocks himself out by taking a slow gentle tumble off the batpod even though he was fine crashing onto a car to save Rachel....

And whatever other plot contrivance you can think of.