Anyone who reviews on a semi-professional basis (ie not necessarily paid, but getting the books for free) is in a difficult position. On the one hand, they obviously want the kudos of author interviews and being at the head of the queue for ARCs of the Next Big Thing or book 17 of That Mega Series, and that really isn't going to happen if they slag off books. On the other hand, their readers want an honest opinion of what a book is like, and any reviewer who 'likes' everything is, frankly, not worth reading. I'll read them sometimes if it's a book I'm really interested in, but I always remember that they're part of the publishing industry. It's advertising, basically.
This varies on the publisher. I have torn books and authors to pieces before but still gotten review copies from publishers. Any publisher that did respond in such a manner would damage risking their own reputation severely.
Still, it is a valid criticism. Some bloggers (such as - I believe - Larry from OF Blog) have in fact withdrawn their name from mailing lists and stopped receiving ARCs to remove this problem. I receive far fewer ARCs now than ever before, with the majority of the books I read and review now being purchases. It might be worth noting which is which on future reviews.
Something that is clear is that book reviewing is in no way as compromised as professional computer games reviewing. Companies like Blizzard, EA and Bethesda pay vast
sums of money to magazines and websites to host adverts for their latest games. Repeatedly, we see games with notable issues (most recently, Mass Effect 3
and Diablo III
) being released which every fan picks up on immediately but which go unmentioned on professional review sites.