What about bookstores? They make recommendations too. Imagine if every used bookstore had to send every book they resold to a lawyer to make sure it didn't defame anyone.
It doesn't appear that they even have a duty to the buyer for it to be a good and useful work.
The duty you want to imagine is real would seem to be impossible to reasonably provide because the cost of paying out every time they were wrong would mean books would all cost $500 and in turn the book stores would all shut down. The only viable way to sell books would be to run a website overseas in a less insane jurisdiction selling epubs.
If you wanted to take it to the next level and enforce liability for anything that goes over the wire you could just give up 99.9% of content in the universe?
In other words: they aren't expected to have actually read every word of every book they sell. Publishers, newspapers, etc, are. But if a book defames me on page 154, the law doesn't hold the bookstore responsible unless it knew. Once it does know, you could probably sue. But that's not the same as treating it like a publisher- it's a different standard.