True, users have to enable Google Drive backups, but they are tied to the user's phone number and

> WhatsApp backups no longer count against your Google Drive storage quota

So I imagine there is some kind of (paid?) agreement between FB and Google to keep them free.

I don't doubt that the content of the messages stays secret, but there is a lot of metadata that can be inferred by simply tying a phone number and the size of the backup (and deltas) to a user's email.