I hate threads like this. In the forum I moderate, we get tons of these shitty threads.
Fact of the matter is, speaking from a mod point of view, there is a reason for everything. We don't just hit a ban button because we're bored. There is always something that points to a person and what other forum members don't understand is, whining about someone being banned wont bring them back any faster. People need to get it through their heads that when the staff bans a member, it isn't anyone's business why. In the forum I mod, and probably here, too, we have a mod/admin section that is hidden on the board so only the staff can see it. We discuss people to watch out for and when we suspect someone of something, we gather evidence and present it in the thread. We then decide the member's fate.
When someone gets banned and people ask why, chances are, you'll just end up pissing off the admin and mods. We make our choices and everyone is expected to live with it. If it was anyone's business as to why a member was banned, we wouldn't make that part of the forum invisible to you all.
It's also not fair to the banned member for us to go around telling people why they were banned. It's embarrassing for them and they also aren't here to defend themselves.
If you want to know why, ask the person directly and if they want to tell you, they will.