If you protect a place like Gotham, it's impossible to do what you set out to do without breaking that rule. Does Batman really think he can keep the Joker off the streets without putting him out of his misery? That clown's best trick is getting out from behind bars. Of any prison. Especially Arkham, which ironically winds up serving as home to his prison cell almost every time he and the Bat fight it out. How often do you hear about the two of them going head to head? How often does the Joker escape? How many more times will it take for Batman to get it through his thick skull that he can't keep locking people like that up?
Sometimes I think the old man gets a thrill out of running into the same old maniacs over and over again. Especially when he gets to play that part out where he's actually genuinely surprised that they've escaped after the millionth time.