Do you give out one star reviews?
I don't have an official policy about it, but I'm not going to struggle through a completely unreadable book just so I can turn around and write a review about how much I hated it. It's not worth the effort. Life is too short for bad books.
It's even pretty rare for me to give out a 2 star rating since it falls into the same category.
Admittedly, this more tends to happen if the book is vastly over-hyped or if it's a continuation of a book series I'm into that should have been allowed to die a dignified and reasonable death a long time ago.