It's not the commercially sound thing to do, but if they're going to keep him around, having Batman rescue him from the legal system is seven kinds of crazy. It fucks up the character something ridiculous.
Joker crippled a woman he sees as a surrogate daughter, murdered his best friend's wife and killed his son and partnerin addition to all the hundreds of people he's sent to horrible deaths just because he was bored on a Tuesday. Stopping himself short of killing him, I can understand. That's borderline superhuman. Stepping and saving him from the clutches of the law that was finally going to give him what he deserves? That's inhuman. That's ignoring the spirit of justice over the letter of the law. That's putting the unsupported belief in a psychotic criminal's redemption above the safety of innocents. In short, it completely justifies every bit of Jason's anger at Batman for letting Joker live, in my opinion.