I'm looking at this interesting ethical quandry in two ways:
1) If the reviewer has perfomed the service (i.e. provided a review), then perhaps the book can be considered compensation for taking the time to do a review.
If that's the case, then the reviewer probably should be able to do whatever they want with it.
2) However, in the absence of a policy that the reviewer agreed to, I believe that if the reviewer does not choose to keep the book, the ethical thing to do might be to contact the sender and ask if they would like it back, or if they would like the reviewer to donate it to a particular group/person.
If the sender doesn't care, then the reviewer should do what they want with it.
If I were in this position, though, I'd keep it. I like the ideas that RPG books generate, even if I don't intend to use them as intended.
As for those who get review copies without doing reviews, see #2.
