To the best of their abilities, they try to convince the reader to buy or not to buy a particular product, and, according to Grice's Maxims (Grice, 1975), they do it in the clearest and most effective way possible.
if an author has in general a positive opinion of a product, the probability of a negative sentence appearing in a review is lower than that of a positive sentence, and even if a negative sentence is introduced, it is likely to appear together with a concession or a contrast marker, such as although or but, or be modified by a hedging expression, such as might, only, could be, which mitigate the negative effect on the reader.