as someone with a strong economic background, it seems obvious to me that an ad reinforces a positive review and a positive review reinforces an ad as far the probability of increasing sales of the product reviewed.
thus, a manufacturer would be smart to place an ad in a publication which has presented a positive review of one of its products.
it also makes sense that a publication with a large enough readership with appropriate demographics would be an ideal venue for an ad, even in absence of a review.
the question asked is there a quid pro quo, before an action is taken, namely if there is coercion to obtain an ad with the inducement of a positive review ?
to the extent that such an event happens, it nullifies the validity of a review.
when you consider what a review is, it is a very subjective and hence highly opinion-based undertaking.
as has been said before, reading them for entertainment purposes probably is the best approach for dealing with issues of ethical lapses.
