Reviews and their origin

I've noticed that sometimes in the reviews it is explicitly stated that LEGO have sent a set for review. Other times no such statement is made.

Is there a hard and fast rule that where LEGO have provided sets for free this is mentioned and hence where its not mentioned the reviewer has purchased the set themselves/on behalf of brickset?

I would like to think that this is the case, transparency/honesty/etc, but was just wondering?