There are no laws requiring that any company must publish all opinions from its customers.
I'll admit it's fundamentally dishonest to only publish the positives, but there's no prohibition against it either. Look at movie advertisements, for example. They'll paraphrase or selectively quote a review, citing only what seems to be positive, yet if you read the review and quote in context, you'll understand the review quite differently.
How exactly would you police it if you were to require companies to publish all
reviews? Would you have some kind of litmus test to determine whether the critique is justified and fair? How often do people post false reviews (good or bad), and how would you account for them?
Company web sites offering reviews form their own customers are doing nothing more than using the web as an extension of their marketing, so one has to expect some manipulation of the information in order to present products and services in their best possible lights. Why, for example, would I tell you how much someone hates my product when my goal is to sell it to you?
This is why it would be better to use independent third-party review sites that have no dog in the hunt when it comes to honest feedback about the companies they cover.