Each time Batman beat, did not kill, and captured the Joker for arrest the judicial system ruled him insane and he was held in a high security psychiatric ward/prison vs killing him. So, using the same measure, you could blame the laws, judges, juries, doctors, etc for all of Jokers future crimes when he broke out agian.
I think the point is that Batman is the only one who can even catch joker to begin with. Plus the legal system in Gotham is rigged so it's not like they wouldn't just let joker leave
Rigged for the rich and powerful right. What the hell kinda influence would a chaos agent like the joker have in such a system. Mobsters I can understand, the joker less so.