Thanks for investigating. I knew about gifting, but I also know that rarely affects the giftee (the gifter is usually responsible for any taxes owed). I'd forgotten about barter, however. Like you, though, I'm not certain that applies here, either.
I figure it's just a case of "bum information" being passed along without any verification. It's not like that never happens.
And if it IS required, then surely there's someone who's sufficiently afraid enough of breaking the law that they DO claim these "free books in exchange for review" and can enlighten us as to how it's done.