The Joker was in prison, there was no reason to suspect anything else happening
people can't just up and say hey we're bringing this dude to your hospital so uh leave a room open k? that is just silly there is procedure involved in these kind of things, procedure that takes more than - what was the time span in that part of the movie? less than 24 hours? sounds about right yeah
The Joker is still an unarmed man; the cops had every reason to assume that a man with a gun was enough to watch him, just like in real life