The whole operation of Ashley Madison - at least what is described of them in wikipedia - is crooked. I have no respect for such an operation that is built on lies and deceit. However, hacking them and dumping their data publicly is illegal regardless of what they are doing (and to the best of my understanding while Ashley Madison did immoral things, they were not illegal things).
If the hackers want to shut down Ashley Madison they might accomplish that, but they have also shown in so doing that there is a market demand for the services they provided, which will just cause someone else - presumably with better network security practices - to launch an identical service.
So fucking what? Slavery used to be illegal, yet we all agree that it wasn't right. And believe me, there definitely was a "market demand" for slaves, at one time.
The older I get, the fewer shits I give about what is "legal". I see law as an instrument of power of the rich and connected. Some of the law is right, some of it isn't. I follow the law so I don't get into trouble, but I sure as hell cheer of this hack.