It's not that believing in redemption is crazy. More specifically, Batman is crazy for believing that redemption is something he has the power to offer. He's offering to try to help fix the Joker, a broken man, when he himself is just as broken. And that is why Batman laughed. Because for that brief moment, he recognized it himself. Another interesting detail that adds even more depth to the joke, are the final 3 panels on that page. Two of them show a beam of light reflected in the water on the ground, and the final panel shows the light going off. My interpretation of this is that this is the moment Batman acknowledges that him offering redemption really is crazy. He understand the meaning behind the joke, and he turns off his 'flashlight', accepting their fates.