Goodreads Librarians have nothing to do with the policies of the site with allowing reviews and ratings early.
Goodreads tries to explain their system here for allowing this, mainly due to ARC's and not wanting to over-police it
https://www.goodreads.com/review/guidelines
Pre-publication reviews. Many of our members receive advance copies of books to review, either through Goodreads giveaways or another source. We have no way of knowing the exact date that review copies are available. As such, each book is eligible to be reviewed as soon as it appears on the site.