Lando's points are correct. There are also many members who join multiple times after being banned. (
userID=53056) for example. He hasn't cheated in his games, but he did with his 10 other accounts. So I have absolutely no trust that he wouldn't do so yet again.
In same cases though, yes people are allowed to keep one account depending on how they reply to moderator inquiries.
Though I suspect what's happening in most of what you see these days is cancelled games. The team is catching cheaters very quickly, before they are able to finish games. We cancel any ongoing games once we ban those cheaters, so all that's left is games they didn't cheat in on their record.