Good question, and all of us in the audiobook industry appreciate you wanting to keep everything on the up and up.
Rest assured that none of the book here are pirated. As a matter of fact, we don't actually have ANY books on our servers (or iffy bittorrent links). When you get a code from our site, you will download it directly from Audible, owned by Amazon.
Ok, so how is that legit? Amazon doesn't just give stuff away!
In this case, they do. When an author, narrator and/or publisher uses the Audible platform called ACX for their audiobooks, the good people there give them free promotional codes to distribute to potential listeners, bloggers, and reviewers to drum up interest. They pay us to list them here and give them out to the thousands of people who come here every week to grab new audiobooks.
So get your free audiobooks here and do the author and narrator a favor and leave a review. They'd appreciate it.