I notice a lot of reviewers buy the sample piece

My understanding is that it could be a combination of both--what is known as an "accommodation" price and a commitment not to resell for a period of time.  I use the word "could" deliberately because I am fairly certain there my be some manufacturers that do not offer accommodation pricing.  Likewise certain reviewers and/or publications might disallow this to maintain journalistic integrity.

They often use that as the ultimate endorsement. I have read in many places that they get a great price, but there is a time limit before they can sell. 

The reviewers get an "endorsement" discount. I feel confident that it is at a cost or below price.