And that's my real issue. Same goes here; users really don't have any idea WHY someone was banned, and don't have access to the full information. Maybe they sent a ton of hugely racist private messages or e-mails that generated complaints against them, just as an example. They're not going to show you those, because it reveals how legitimate the ban is, and there's no way for anyone else to have access to that, so there's no public "evidence" to "justify" the ban.
But they don't HAVE to justify those bans to the public. And it would be a breach of privacy policies to do so, anyway.