There are usually three reasons why anyone works for The Joker that are commonly accepted by fans.
1. Money. Despite the fact that he's nutty as a fruitcake, the fact is that when he decides to actually commit a crime with some form of gain, he is one hell of a thief, and has more than enough cash lying around to pay henchmen buttloads. And there are some people out there who would, to varying degrees, be willing to look the other way when they see all those unmarked bills.
2. Hero worship. For whatever reason, they think he's the hottest thing since flamethrowers were invented. Impressionable minds that either he has manipulated, or have come to him on their own, who look at him almost as a god or messiah figure. Sort of a Manson Family thing going on. Maybe they see him as a philosopher terrorist who sticks it to the man who desperately needs sticking to. Maybe they dig the fact that he has completely abandoned any notions or morality or sanity, as they see them as hindering and outdated. And maybe they think that he's simply a madman who acts on behalf of the greater truth of madness itself. In any event, they think he's bigger than sliced bread.
3. Madness. Something of an extension of the hero worship, as some of his groupies are probably impressionable because they're kind of crazy. Basically, the henchmen here are about as crazy as The Joker is, or crazy enough in a different way that they simply don't see the problem with what he does. They work with him because they think like him, or even if they don't, think differently enough from everyone else that they have an easier time co-existing with The Joker.
