I received the same email from Talent Smart. I took great offense to this as it calls into question their ethics. In response I posted an Amazon review and repeated comments disclosing this practice. I have been contacted by Amazon asking me to not repeat my post against recent 5 star reviews. Repeated comments are a violation of their policy - even if, evidently, they contain factual information that buyers might want to consider. I have asked that they implement policies that protect the integrity of the rating policy. I hope you will add your voice to this so that we can confidently rely on the opinions of others as being authentic and not manipulated by offers to receive free goods or services.
The problem is offering a tangible incentive in return for a 5 star review. They are not simply asking that people post reviews with their opinion. They are explicitly saying they will exchange a free signed copy or assessment in return for posting a 5 star review (the offer is not valid if you post anything less.) The reviews are tainted as a result as they are simply paid advertisements, not the authentic unbiased opinions one would expect from an Amazon review board.