There are banned individuals that a part of me would like to say something like, "Why did you ban so-and-so! What could they have possibly done that would deserve getting banning?!"; I'm probably sure in most cases the staffs response, if they made one hypothetically that is, would be something akin to, "You don't even want to know bro".
So as much as it may suck having the moment where you try to PM someone, then see they have been banned, and you can't think of a reason why they were banned. That's likely much better for everyone than any possible alternatives.