It's their content. They can do anything they like.

Sort of. There are countries like France where attribution rights are fundamentally inalienable, and the author can always demand attribution, even after forever waiving the right to attribution. But in the US, the party who benefits from a contractual provision can generally choose to ignore or forfeit that provision. Whether it's still called the "CC BY" after that is debatable, since the whole point is attribution, but licensing by the CC BY and immediately waiving that requirement seems legitimate to me.