Unless something changed recently, every title on Downpour is DRM free when bought (as opposed to rented). I've been using Downpour for more than a decade and own tons of books. Libro.fm is slightly newer and IMO has slightly nicer UX, but both websites have mostly the same (wide) selection of titles.

I can't tell you why publishers make the decisions they do, but there's no trick here, if that's what you're asking. DRM free audio books are widely available and have been widely available for a long time now.

The real question is, why does Audible insist on putting DRM on their Audiobooks when the publishers clearly don't care? I don't know the answer to that either, but the upshot is that everyone should stop buying from Audible!