Not to rationalize it, but it appears that they're gatekeeping the dataset to get access to the OCR-scans from the people they choose to share it with. This is to improve their existing service by making the content of books (and not just their title/tags) searchable.
As per the blog post:
>What does Anna’s Archive get out of it? Full-text search of the books for its users.
Fair enough, it just seems like they're painting an even bigger target on their backs by restricting access to copyrighted material they don't own the rights to
As per the blog post: >What does Anna’s Archive get out of it? Full-text search of the books for its users.