You don't have to spend long on my site to see that all my reviews are overwhelmingly positive. It's because I don't post negative reviews. Ever.
There are so many reasons for me to only post positive reviews. The most obvious and selfish reason is that I'm running a book-selling business here. I'd be shooting myself in the foot if I were to tell you what not to read.
But the real reason is, I don't want to stand between a reader and a book, not matter how bad I think the book is.
Tastes are so subjective; just because I hated it doesn't mean you will, and it would be a shame if I discouraged someone from reading what could have been their favorite book.
So often, I'll hate a book and be baffled that anyone would enjoy it. Then, I'll talk to someone who loved it, and while the conversation might not change my mind about the book, at the very least I can learn to appreciate what I didn't understand before. We should be having conversations to help us understand what matters to other people, not telling others why the things that matter to them are actually garbage.
Another reason I don't post bad reviews: authors are people. In fact, many of my friends are published novelists. I might feel comfortable publicly shooting down a product, but I am not comfortable shooting down another person's creative work. I also like to practice good karma; someday, I hope others will review my books, and I don't want the universe to have a purse full of negativity that it's been saving up just for me.
Trust me, I can firmly attest that there are some bad books out there. But I don't think we should handle bad books by announcing to everyone how terrible they are. I think we should just not talk about them at all because we're so busy going on and on about all the books we loved, and then all the mediocre books will be forgotten.