Yeah, that's been a problem with reviews for a long time. In fact it's what Consumer Reports used initially to differentiate themselves: their "thing" was that they only reviewed products bought anonymously at retail (no free samples or manufacturer-provided review items) and didn't accept any advertising from manufacturers either.
Sites that receive free review samples and are supported by affiliate links are kind of the exact opposite model.
It does in a funny way provide something of a metric for how willing the site is to be critical. Several video game reviewers I follow have stopped receiving product from some studios, which I think is a badge of honest review. Although it's not something you'd know easily so it doesn't help much in terms of finding good reviewers.
Sites that receive free review samples and are supported by affiliate links are kind of the exact opposite model.